Shape memory alloy microfilament yarns

ABSTRACT

Shape memory yarns described herein include twisted microfilaments made from a shape memory alloy that may provide superelastic or shape memory properties. The shape memory yarns are formed into coils that provide a high degree of actuation or elasticity along an axis of the coiled shape memory yarn, and may have relatively low porosity, low rigidity, and/or low change of volume compared to shape memory coils formed from solid structures. Coiled shape memory yarns may provide further tailorability of a superelastic or shape memory response of a system or device incorporating the coiled shape memory yarns through various coil parameters, such as coil pitch or density, or torque balancing, such as heat treating or plying the coiled shape memory yarns.

This application is a PCT application claiming priority to U.S. Provisional Patent Application No. 63/077,350, filed Sep. 11, 2020, the entire contents of which is incorporated herein by reference.

TECHNICAL FIELD

The present disclosure relates to shape memory alloy microfilament yarns.

BACKGROUND

Textiles that incorporate multifunctional materials may provide enhanced properties to devices, articles, or systems formed from the textiles. These enhanced textiles may be used in a variety of applications, including medical devices, wearable electronics, energy absorption applications, and thermal insulation. As one example, textiles may incorporate thermoelectric, piezoelectric, or triboelectric materials to provide energy harvesting functionality for wearable electronics, such as sensors, attached to the textiles. For textiles that incorporate relatively simple, solid material structures, such as wires and rods, properties of the materials, such as flexural rigidity, may be limited to bulk properties of the material alone or as incorporated into the textile.

SUMMARY

The present disclosure describes coiled shape memory yarns that provide enhanced and tailorable properties to textiles, actuators, and other materials and devices that incorporate the coiled shape memory yarns.

Shape memory yarns described herein include twisted microfilaments made from a shape memory alloy, such as nitinol, that may provide superelastic or shape memory properties. Mechanical properties of the shape memory yarns, such as structural stiffness, plateau strength, and cyclical consistency, can be customized by configuring various structural parameters of the shape memory yarns, such as an amount of twist, a density, and/or a packing efficiency. The shape memory yarns are formed into coils that provide a high degree of actuation or elasticity along an axis of the coiled shape memory yarn, and may have relatively low porosity, low rigidity, and/or low change of volume compared to shape memory coils formed from solid structures. Coiled shape memory yarns may provide further tailorability of a superelastic or shape memory response of a system or device incorporating the coiled shape memory yarns through various coil parameters, such as coil pitch or density, or torque balancing, such as heat treating or plying the coiled shape memory yarns.

The coiled shape memory yarns can be incorporated into textiles and actuators to provide superelastic functionality, such as energy absorption and/or passive strain recovery, or shape memory functionality, such as active strain recovery and actuation, in a variety of applications. As one example, an actuator that incorporates coiled shape memory yarns may provide a relatively high actuation force or displacement while maintaining relatively small volumetric changes and being responsive to relatively low actuation temperatures. As another example, a textile that incorporates coiled shape memory yarns may provide high strain recovery with low structural rigidity and porosity.

In one example, a shape memory coil includes a coiled shape memory yarn having a coil direction around a coil axis. The coiled shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.

In one example, a method for manufacturing a shape memory coil includes coiling a shape memory yarn to form a coiled shape memory yarn that has a coil direction around a coil axis. The coiled shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.

In one example, a textile includes a plurality of interlocked tows. At least a portion of the plurality of interlocked tows includes a plurality of shape memory yarn structures. Each shape memory yarn structure includes one or more shape memory yarns. Each shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy.

In one example, a device includes one or more shape memory yarn structures and a current source coupled to the one or more shape memory yarn structures. Each shape memory yarn structure includes one or more shape memory yarns. Each shape memory yarn includes a plurality of microfilaments having a twist direction around a yarn axis. The plurality of microfilaments includes a shape memory alloy. The shape memory alloy is configured to undergo a phase transformation in response to heating above a transformation temperature. The current source is configured to send an actuation signal to the one or more shape memory yarn structures to heat the one or more shape memory yarn structures above the transformation temperature of the shape memory alloy.

The details of one or more examples of the disclosure are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the disclosure will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a diagram illustrating an example textile incorporating shape memory yarns.

FIG. 1B is a diagram illustrating an example actuation system incorporating shape memory yarns.

FIG. 2A is a side view diagram illustrating an example coiled shape memory yarn.

FIG. 2B is a side view diagram illustrating an example homochiral shape memory yarn.

FIG. 2C is a side view diagram illustrating an example heterochiral shape memory yarn.

FIG. 2D is a top view diagram illustrating an example textile having woven tows of coiled shape memory yarn.

FIG. 2E is a top view diagram illustrating an example textile having knitted loops of coiled shape memory yarn.

FIG. 3A is a flowchart of an example method of forming a shape memory yarn.

FIG. 3B is a flowchart of an example method of forming a homochiral shape memory yarn.

FIG. 3C is a flowchart of an example method of forming a heterochiral shape memory yarn.

FIG. 4A is a SEM image of clusters of NiTi microfilaments in a sacrificial iron matrix.

FIG. 4B is a SEM image of two clusters of NiTi microfilaments encased in an iron matrix.

FIG. 4C is a SEM image of NiTi microfilaments after etching of an iron matrix.

FIG. 4D is a SEM image of a surface of a single NiTi microfilament with residual draw lines and iron particles.

FIG. 5A is a cross-sectional optical image of a NiTi microfilament bundle before twist insertion.

FIG. 5B is a diagram of a yarn spinning process of a bundle of untwisted filaments through a delivery and spindle system of an industrial ring spinner into a twist inserted single yarn.

FIG. 5C is a SEM image of a twist inserted NiTi microfilament single yarn.

FIG. 5D is a diagram of a yarn plying process of multiple single twisted yarns spun together with an opposite twist direction.

FIG. 5E is a SEM image of a NiTi microfilament two-ply yarn structure in a bent loop orientation.

FIG. 5F is a diagram of a knit manufacturing process of a yarn structure being fed through a series of needles capable of manipulating the yarn into interlacing loops.

FIG. 5G is a SEM image of a section of a knitted textile manufactured with NiTi microfilament yarn.

FIG. 6A is a graph of isothermal uniaxial testing results through a descending sweep of temperatures.

FIG. 6B is a graph of uniaxial testing results for three NiTi microfilament single yarns spun at different twist levels.

FIG. 6C is a graph of uniaxial testing results for NiTi microfiber two-ply yarn variants with different levels of twist.

FIG. 6D is a graph of uniaxial testing results comparing austenitic loading profiles of a NiTi microfilament single yarn to a similar spun NiTi microfilament two-ply yarn.

FIG. 6E is a graph of uniaxial testing results comparing a torque balanced NiTi microfilament single yarn to a torque imbalanced NiTi microfilament single yarn.

FIG. 6F is a graph of cycled loading results for a NiTi microfilament 3.26 TPCM single yarn structure.

FIG. 7A is a strain-force graph illustrating a 3.51 TPCM torque balanced single yarn that recovers a substantial portion of deformations using the shape memory effect.

FIG. 7B is a strain-force graph illustrating a 7.03 TPCM torque balanced single yarn that partially recovers deformations using the shape memory effect.

FIG. 7C is an actuation-force graph illustrating the 3.51 TPCM yarn that provides a maximum actuation contraction under a 10 N applied load while the 7.03 TPCM yarn provides a maximum actuation contraction at 5 N.

FIG. 7D is a strain-force graph illustrating the 3.51 TPCM yarn that provides higher forces in both martensite and austenite when stretched by the same amount.

FIG. 7E is a strain-force graph illustrating a 7.03 TPCM yarn.

FIG. 7F is an actuation-force graph illustrating both the 3.51 TPCM and 7.03 TPCM single yarn for force blocking tests from 2% to 8%.

FIG. 8A is a photograph of a microfilament NiTi textile when loaded at room temperature.

FIG. 8B is a photograph of the microfilament NiTi textile when released at room temperature.

FIG. 8C is a photograph of the microfilament NiTi textile when loaded at temperatures above AF of the constituent NiTi microfilaments.

FIG. 8D is a photograph of the microfilament NiTi textile when released at temperatures above AF of the constituent NiTi microfilaments.

FIG. 8E is a photograph of a microfilament NiTi textile when unloaded at room temperature.

FIG. 8F is a photograph of the microfilament NiTi when heated above AF.

FIG. 9A is a diagram of an over-twisted coil yarn manufacturing system.

FIG. 9B is a diagram illustrating four main manufacturing results with a direct relationship to the amount of stress applied to the NiTi bundles.

FIG. 9C is a graph of DSC results for pre- and post-processed NiTi material.

FIG. 9D is a SEM image of a 800-10 over-twisted coiled yarn.

FIG. 9E is a SEM image of a plied 800-10 over-twisted coiled yarn.

FIG. 9F is a SEM image of a 800-10 over-twisted coiled yarn spring.

FIG. 9G is a SEM image of a loop made from 400-10 over-twisted coiled yarn.

FIG. 10 is a graph summarizing the manufacturing study.

FIG. 11A demonstrates a typical curve for a 400-10 bundle configuration during coil manufacturing.

FIG. 11B demonstrates a typical curve for a 800-10 bundle configuration during coil manufacturing.

FIG. 11C demonstrates a typical curve for a 1200-10 bundle configuration during coil manufacturing.

FIG. 11D demonstrates a typical curve for a 1600-10 bundle configuration during coil manufacturing.

FIG. 12A is a SEM image of 800-10 over-twisted coiled yarn to define the geometric parameters used to describe the final form of the coiled yarn.

FIG. 12B is a graph of average geometric parameter values measured from SEM images on four bundle configurations with increasing amounts of filaments.

FIG. 13A is a graph of preliminary isothermal uniaxial displacement-controlled test results on an 800-10 coiled yarn.

FIG. 13B is a graph of uniaxial testing on an 800-10 coiled yarn at temperatures above AF and below MF.

FIG. 13C is a graph of cycled uniaxial testing of an 800-10 coiled yarn.

FIG. 14A is a graph of full structural strain and force curves for free displacement testing of an 800-10 coiled yarn.

FIG. 14B is a graph of actuation contraction results from the test illustrated in FIG. 14A.

FIG. 14C is a graph of free displacement testing results to quantify the actuation contraction potential of an 800-10 OTC yarn.

FIG. 14D is a graph of force blocking testing results that include scalable generated forces exhibited through manipulation of bundle configurations in a 400-10, 800-10, and 3200-5 OTC yarns.

FIG. 14E is a graph of specific damping capacity at various oscillation amplitude strains.

FIG. 14F is a graph of specific damping capacity at various oscillation frequencies.

FIG. 15A is an image of manufacturing of an 800-10 OTC yarn woven textile in a loom.

FIG. 15B is a cross-sectional diagram of a woven structure in which OTC yarns are interwoven by Kevlar thread.

FIG. 15C is an image (left) of an OTC yarn woven textile amidst manufacturing, and a close-up image (right) of interaction between OTC yarn and Kevlar within the woven.

FIG. 15D is an image of manufacturing of a 400-10 OTC yarn garter knit textile on a weft knitting machine.

FIG. 15E is a diagram of a unit loop of an OTC yarn garter knit to demonstrate the path of a 1D over-twisted coiled yarn within a larger structure.

FIG. 15F is an image (left) of a final garter patterned knitted textile composing of 400-10 OTC yarn, and a close-up image (right) of interaction between the unit loops.

FIG. 16A is a graph of force block results of the 800-10 OTC yarn woven textile.

FIG. 16B is a graph of force block results of the 400-10 OTC yarn knitted textile.

FIG. 16C is a graph of the generated force in the woven textile, knitted textile, and constitutive 1D 800-10 and 400-10 OTC yarns.

FIG. 17A is diagram of a garter knitted textile composed of SMA microfilament yarns for applications in active compression garments applications.

FIG. 17B is a diagram of a 3D spacer textile with Kevlar knitted surfaces and SMA microfilament spacer yarn for potential use in prosthetic socket liner applications.

FIG. 18A is a graph of force block results of the cloth-like SMA knitted garter textile.

FIG. 18B is a graph of generated forces from the thermal heating ramps of the cloth-like SMA knitted garter textile.

FIG. 18C is a graph of isothermal quasistatic compression results of the spacer textile at a temperature above A_(F) and below M_(F).

FIG. 18D is a graph of oscillatory results of the spacer textile at varying pre-strains and oscillation amplitude strains.

FIG. 19A is an image (left) of a spacer fabric manufacturing process, and a diagram (right) of the spacer yarn pattern integrated across two Kevlar stockinette surfaces.

FIG. 19B are images of (left) a side view and (right) a top view of a finished spacer fabric.

FIG. 20A is a graph of a loading curve of the NiTi coiled yard spacer fabric in austenite phase, including an initial engagement stage (I), a structural reorientation stage (II), and a densification stage (III).

FIG. 20B is a graph of loading and unloading curves of NiTi coiled yarn spacer fabric in austenite and martensite phases.

FIG. 20C is a graph of oscillation test results on the NiTi coiled yarn spacer fabric illustrating the relationship between oscillation frequency and structural prestrain on the tan delta.

FIG. 20D is a graph of oscillation test results on the NiTi coiled yarn spacer fabric illustrating the relationship between oscillation frequency and structural prestrain on the loss modulation.

DETAILED DESCRIPTION

Systems and devices described herein utilize shape memory yarns to provide enhanced and tailorable properties to articles, devices, or systems that incorporate the shape memory yarns, such as textiles or actuators. Shape memory yarns may be used in a variety of technical fields including, but not limited to: medical devices, such as sutures, grafts, stents, tissue engineering scaffolds, grippers, body navigation aids, drug delivery aids, and compression garments; clothing, such as athletic garments, shoes, insoles, and sports equipment; household devices, such as filters; defense applications, such as impact resistant or energy absorbing wearables; aerospace applications, such as active skin topologies; and the like. FIGS. 1A and 1B below describe two example textiles that incorporate shape memory yarns to utilize a superelastic and/or shape memory effect of the shape memory yarns as compression garments and actuators, respectively; however, the yarns, textiles, and actuators described herein may be used in other fields, including any of the fields described above.

In some examples, shape memory yarns described herein may be incorporated into passive or active energy absorption textiles. FIG. 1A is a diagram illustrating an example device 10 that includes a textile 12 that incorporates shape memory yarns to provide a superelastic or shape memory effect. In the example of FIG. 1A, device 10 is illustrated as a compression sleeve for an appendage of a patient. For example, device 10 may provide passive or active strain recovery in one or more directions, such that device 10 may provide a compressive radial force on the appendage of the patient that may be actuated through heat or recovered upon unloading.

Textile 12 includes a plurality of interlocked structures, such as woven tows, knitted loops, or other interlocked structures. At least a portion of the plurality of interlocked structures include a plurality of shape memory yarn structures 14A and 14B (collectively, “shape memory yarn structures 14”). For example, the plurality of shape memory yarns structures 14 may be mixed with other types of fibers to provide a desired elastic or shape memory effect combined with other textile properties. Each shape memory yarn structure 14 includes one or more shape memory yarns. For example, shape memory yarn structure may include a single yarn, or may include a multiple ply yarn. Each shape memory yarn includes a plurality of microfilaments that include a shape memory alloy. The shape memory alloy may be configured to provide enhanced elasticity and/or actuation in one or more directions.

In some instances, textile 12 may be configured to provide different properties along different axes of textile 12. For example, the plurality of shape memory yarn structures 14 includes a first plurality of shape memory yarn structures 14A oriented in a first direction and a second plurality of shape memory yarn structures 14B oriented in a second direction, different from the first direction. The first and second pluralities of shape memory yarn structures 14A and 14B may be configured with different structural parameters and/or at different densities (e.g., proportions with other, non SMA fibers), such that textile 12 may have different properties in the first and second directions. As one example, first plurality of shape memory yarn structures 14A may have a first elasticity and second plurality of shape memory yarn structures 14B may have a second elasticity, different from the first elasticity, such as for textiles that resist force from a particular direction. As another example, first plurality of shape memory yarn structures 14A may be configured to produce a first actuation force in response to a transformation temperature transition and second plurality of shape memory yarn structures 14B may be configured to produce a second actuation force in response to a transformation temperature transition, different from the first actuation force, such as for textiles that provide force primarily to a particular direction.

In some instances, shape memory yarn structures 14 may incorporate coiled shape memory yarns. As will be described further below, textile 12 that incorporates coiled shape memory yarns may provide high strain recovery with low structural rigidity and porosity. For example, coiled shape memory yarns may be relatively close packed, such that textiles formed from coiled shape memory yarns may have a relatively low porosity. As another example, coiled shape memory yarns may be relatively flexible compared to solid shape memory alloy rods or coils, such that textiles formed from coiled shape memory yarns may have high drapability and conformability to surfaces, such as body parts.

Various structural parameters of textile 12 may provide enhanced properties of textile 12 over individual yarn structures 14. In some examples, textile 12 may have a higher structural strain compared to equivalent shape memory yarn structures 14, as illustrated in FIG. 16C below. Without being limited to any particular theory, a woven or knitted textile may include a plurality of yarn structures 14 in parallel to share a total applied load relatively evenly across each yarn structure such that, as heavily loaded yarn structures weaken, the load may be transferred to adjacent yarns structures that were previously unloaded. Such load sharing allows textile 12 to be strained to large structural strains beyond that of a single equivalent yarn structure. In some examples, a shape of tows within textile 12 may be selected for various enhanced properties of textile 12. For example, textile 12 may include knitted loops, such as shown in FIG. 15E below, that convert structural displacement to reorientation of the constitutive loops, in addition to bending, torsional, and axial stressing of yarn structures 14 to increase allowable structural strains.

Various structural parameters of textile 12 may be configured to provide particular bulk properties of textile 12. For example, tows, loops, or other shapes of yarn structures 14 may be configured in a manner that enables reconfiguration of load-bearing yarn structures 14 and load sharing of yarn structures 14, such that a force profile (e.g., a generated force over a structural strain) may be tailored for a particular textile 12. In some examples, textile 12 may be configured to have a particular force profile. For example, textile 12 having woven tows may be configured to generate large forces at relatively low structural strains. As another example, textile 12 having knitted loops may be configured to generate large forces at relatively high structural strains, such as due to reorientation of yarn structures to mitigate the strains and stresses applied directly to yarn structures 14. In some examples, textile 12 may include portions having different force profiles, such as a first portion that includes a first textile pattern configured to generate a force at a relatively low structural strain and a second textile pattern configured to generate a force at a relatively high structural strain.

In some examples, shape memory yarns described herein may be incorporated into actuators. FIG. 1B is a diagram illustrating an example device 20 that includes an actuator 22 that incorporates shape memory yarns to provide actuation. In the example of FIG. 1B, device 20 is illustrated as a linear actuator device for creating an actuation force. Actuator 22 is formed, at least partly, from a textile 26. Textile 26 includes one or more shape memory yarn structures 28. Each shape memory yarn structure 28 includes one or more shape memory yarns, and each shape memory yarn includes a plurality of microfilaments that include a shape memory alloy. Each shape memory yarn structure 28 is configured to transform between a martensitic state and an austenitic state in response to a change in temperature.

Device 20 includes a current source 24 coupled to the one or more shape memory yarn structures 28. Current source 24 is configured to send an actuation signal to the one or more shape memory yarn structures 28 to heat the one or more shape memory yarn structures 28 above a transformation temperature of the shape memory alloy. In response to being heated above the transformation temperature, the one or more shape memory yarn structures 28 may transform from a first state to a second state, with a corresponding change in dimensions, to create the actuation force. In response to cooling below a transformation temperature, the one or more shape memory yarn structures 28 may transform from the second state back to the first state.

In some instances, shape memory yarn structures 28 may include coiled shape memory yarns. As will be described further below, actuator 22 that incorporates coiled shape memory yarns may provide a relatively high actuation force and/or displacement while maintaining relatively small volumetric changes and being responsive to relatively low actuation temperatures. As such, volume-constrained and temperature-constrained fields, such as artificial muscles, may incorporate coiled shape memory yarns to provide actuation that is compatible with human tissues.

FIG. 2A is a side view diagram illustrating an example shape memory coil 40. Shape memory coil 40 is formed from a shape memory yarn 42, and includes a coiled portion 44 that includes one or more coils and a straight portion 46 that is substantially free of coils. However, in some instances, shape memory coil 40 may include only coiled portion 44, or may include multiple coil portions 44 having different structural or compositional parameters. Shape memory yarn 42, and coiled portion 44 of shape memory coil 40, may be configured with various structural and compositional parameters to provide shape memory coil 40 with a desired behavior.

Shape memory coil 40 may exhibit shape memory behavior, such that shape memory coil 40 may be configured to transform between a martensitic state and an austenitic state in response to a change in temperature. For example, shape memory coil 40, shape memory yarn 42, and/or microfilaments of shape memory yarn 42 may be loaded and heat treated at a temperature at or below a martensitic transformation temperature of the shape memory alloy. As a result of this heat treatment, shape memory coil 40 may be configured to transform from the martensitic phase to the austenitic phase when heated above an austenite finish temperature. The change in lattice structure from the phase transformation enables recovery of plastic and elastic strains to a memorized austenitic shape, as set by the heat treatment. This transformation may create an actuation force along a coil axis 56.

Shape memory coil 40 may exhibit superelastic behavior, such that shape memory coil 40 be configured to transform between a martensitic state and an austenitic state in response to mechanical stress. For example, any of shape memory coil 40, a device or textile formed from shape memory coils 40, shape memory yarn 42, and/or microfilaments of shape memory yarn 42 may be loaded and heat treated at a temperature at or above an austenitic transformation temperature of the shape memory alloy. As a result of this heat treatment, shape memory coil 40 may be configured to transform from the austenitic phase to a stress-induced martensitic phase upon loading, and hysterically recover back to the austenitic phase upon unloading. This transformation may enable shape memory coil 40 to have a recoverable form upon unloading.

Coiled portion 44 of shape memory yarn 42 defines coil axis 56, along which shape memory coil 40 may expand or contract. While shown as being straight, coil axis 56 can have a variety of shapes, such as a curved or multi-directional shape. Coiled portion 44 of shape memory yarn 42 also defines a coil direction 58 around coil axis 56. Coil direction 58 may represent a rotational direction of coiled shape memory yarn 42 around coil axis 56. For example, in FIG. 2A, coil direction 58 is clockwise when viewed along coil axis 56. Coiled portion 44 may have a coil diameter 52. Coil diameter 52 may represent a radial length of coiled portion 44 perpendicular to coil axis 56. While shown as being constant, coil diameter 52 may vary throughout shape memory coil 40, such as in various coiled portions having different heat treatments, different local tension, different yarn diameters 48, and other factors that may affect an amount of deformation of coiled shape memory yarn 42. In some examples, coil diameter 52 is between about 20 micrometers and about 100 micrometers.

Each coil in coiled portion 44 may be separated by a coil spacing 54 that defines a coil pitch. In examples in which shape memory coil 40 is tightly packed, as shown in FIG. 2A, coil spacing 54 may be limited by diameter 48 of coiled shape memory yarn 42. In examples in which shape memory coil 40 includes loosely spaced coils, coil spacing 54 may be based on various forces used in manufacturing, such as will be described in FIG. 3B below. In some examples, coil spacing 54 is between about 5 micrometers and about 100 micrometers. In some examples, the various parameters of shape memory coil 40, such as coil diameter 52, coil spacing 54, and coil direction 58, may be configured to provide various properties to shape memory coil 40.

Shape memory yarn 42 has a yarn axis 50 that follows a center of shape memory yarn 42 through shape memory coil 40. Within coiled portion 44, yarn axis 50 forms an acute coil angle 51 with coil axis 56. Shape memory yarn 42 has a diameter 48 that represents a radial length of shape memory yarn 42 perpendicular to yarn axis 50. While shown from a side view as being straight, yarn axis 50 may be curved along coil portion 44 and straight along straight portion 46.

FIG. 2B is a side view diagram illustrating an example shape memory yarn 42A of a homochiral shape memory coil 40. Shape memory yarn 42 includes a plurality of microfilaments 60. The plurality of microfilaments 60 may include any number of microfilaments. In some examples, the plurality of microfilaments 60 includes between about 10 and about 1000 microfilaments. Each of the plurality of microfilaments 60 may be relatively small, such that shape memory yarn 42 has a relatively low rigidity compared to yarns that include larger filaments. In some examples, the plurality of microfilaments 60 has an average diameter less than about 10 micrometers, such as between about 1 micrometer and about 10 micrometers.

The plurality of microfilaments 60 are formed from a shape memory alloy. A shape memory alloy may include any alloy configured to undergo a reversible phase transformation in response to a change in temperature or mechanical stress. In some examples, the shape memory alloy includes at least one of a nickel-titanium alloy, such as nitinol, or a copper-zinc-aluminum alloy.

Each microfilament 60 defines a microfilament axis 62 that forms a twist angle 64 with yarn axis 50 and a twist direction 66A around yarn axis 50. The plurality of microfilaments 60 may be characterized by an amount of twist, such as a number of turns per centimeter (TPCM). A geometry of shape memory yarn 42 may be characterized by a yarn count (or linear density), a yarn twist, and a packing factor, that together influence twist angle 64. Yarn count may represent an amount of active material within shape memory yarn 42, and may be related to a density of the plurality of microfilaments 60. Yarn twist may represent a degree of twist in shape memory yarn 42, and may be related to an amount of twist of individual shape memory yarns 42 and a number of plies of shape memory yarn 42 (not shown). Packing factor may represent a specific volume of the plurality of microfilaments 60 to a specific volume of shape memory yarn 42, and may be related to a size of the plurality of microfilaments 60.

In some examples, various structural parameters of shape memory yarn 42, such as yarn diameter 48, twist angle 64, a density of microfilaments 60, or a number of plies (not shown in FIGS. 2A-2C) may be configured to provide various properties to shape memory yarn 42 and/or shape memory coil 40. For example, as will be illustrated in FIGS. 7A-7F, 8A-8F, and 9A-9F, a structural stiffness of shape memory yarn 42, a plateau strength of shape memory yarn 42, or cyclical consistency of shape memory yarn 42 may be tailored by selecting the various structural and compositional properties of shape memory yarn 42. As one example, as an amount of twist increases, as characterized by increasing twist angle 64, a stiffness and strength of shape memory yarn 42 and/or shape memory coil 40 may decrease. As another example, as a number of plies of shape memory yarn 42 increases (not shown in FIGS. 2A-2C), a plateau strength of shape memory yarn 42 and/or shape memory coil 40 may increase.

Shape memory coil 40 may be configured to have a homochiral or heterochiral configuration, or a mixture thereof. In the example of FIG. 2B, with respect to FIG. 2A, coil direction 58 of shape memory yarn 42A and twist direction 66A of the plurality of microfilaments 60 are the same, such that shape memory coil 40 that includes shape memory yarn 42A may be homochiral. A homochiral configuration may form a relatively close packed coiled portion 44, such as shown in FIG. 2A. FIG. 2C is a side view diagram illustrating an example shape memory yarn 42B of a heterochiral shape memory coil 40. In the example of FIG. 2C, with respect to FIG. 2A, coil direction 58 of a shape memory yarn 42B and a twist direction 66B of the plurality of microfilaments are different, such that shape memory coil 40 that includes shape memory yarn 42B may be heterochiral.

In some examples, various heat treatment parameters of shape memory yarn 42 and/or shape memory coil 40 may be configured to provide various properties to shape memory yarn 42 and/or shape memory coil 40. As one example, as a heat treatment temperature of shape memory yarn 42 and/or shape memory coil 40 increases, a stiffness and/or plateau strength of shape memory yarn 42 and/or shape memory coil 40 may increase. As another example, heat treatment of the plurality of microfilament 60 prior to forming shape memory yarn 42 and/or shape memory coil 40 may result in residual fiber stresses after twisting and/or coiling, while heat treatment of the shape memory yarn 42 and/or shape memory coil 40 may result in relieved fiber stresses after twisting and/or coiling.

In some examples, shape memory coil 40 may be configured to have particular damping properties. For example, superelastic shape memory yarns 42 may exhibit a relatively large hysteresis through the forward and reverse transformations between austenitic and martensitic phases. The hysteresis, which is mechanical energy that is absorbed and dissipated as heat, can be leveraged in damping applications. In some examples, shape memory coils 40 may have a relatively high specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), compared to shape memory yarns 42. This damping behavior may be tailored by controlling an oscillation amplitude strain, pre-strain, yarn/coil thickness, yarn/coil pattern, and yarn/coil configuration of the shape memory coils 40.

Shape memory yarn 42 may be formed into a variety of textiles. In some examples, various parameters of the textiles may be selected to produce a textile having particular bulk properties. For example, a force profile (e.g., generated force across structural strain) may be produced by a textile based on a shape of shape memory yarn 42 and interaction of shape memory yarn 42 with adjacent shape memory yarns in the textile.

In some examples, shape memory coil 40 may be formed into a textile of interlocked tows having a relatively stable orientation between shape memory coils 40, such as woven tows. FIG. 2D is a top view diagram illustrating an example textile having woven tows 68 of shape memory coil 40. Textiles having woven tows 68 may produce a force profile having a relatively quick onset of generated force and a relatively low structural strain (though higher than an equivalent textile of individual shape memory coils 40). For example, each shape memory coil 40 may be configured to deform and exert an actuation force in a single direction along an axis of the tow, such that adjacent woven tows 68 tows may not substantially reconfigure in response to a force.

In some examples, shape memory coils 40 may be formed into a textile of interlocked tows having a reconfigurable orientation between shape memory coils 40, such as knitted loops. FIG. 2E is a top view diagram illustrating an example textile having knitted loops 69 of shape memory coil 40. Textiles having knitted loops 69 may produce a force profile having a relatively slow onset of generated force as loops 69 reconfigure within the textile and a relatively high structural strain as a load is distributed over a greater displacement. For example, each shape memory coil 40 may be configured to deform and exert an actuation force in a multiple directions around the loop, such that adjacent knitted loops may substantially reconfigure in response to a force.

FIG. 3A is a flowchart of an example method of forming any of a shape memory yarn, such as shape memory yarn 42 of FIG. 2A, a shape memory coil, such as shape memory coil 40 of FIG. 2A, or a textile, such as textile 12 of FIG. 1A or textile 24 of FIG. 1B. The method of FIG. 3A will be described with respect to FIGS. 1A and 1 i, and FIGS. 2A-2C.

The method of FIG. 3A may include forming a plurality of microfilaments (70), such as plurality of microfilaments 60 of FIG. 2B. In some examples, plurality of microfilaments 60 may be formed by an accumulative drawing/rolling and bonding technique (72), such as that described in the Microfilament Manufacturing portions of the Experimental Section below. A shape memory alloy, such as nitinol, may be drawn with a sacrificial matrix, such as an iron matrix, using a wire drawing process. As a result, multiple microfilaments may be organized into a cluster, and multiple clusters may be organized into a bundle. The sacrificial matrix may be chemically removed to reveal the plurality of microfilaments 60. In this way, microfilaments having relatively small diameters, such as less than about 10 micrometers, may be formed. In some examples, plurality of microfilaments 60 may be heat treated (74). For example, to increase phase transition stability or create a phase transition temperature shift (e.g., martensite finish temperature), the plurality of microfilaments 60 may be heat treated prior to forming yarns from the plurality of microfilaments 60.

The method of FIG. 3A may include forming shape memory yarn 42 from the plurality of microfilaments 60 (80). Forming shape memory yarn 42 includes twisting the plurality of microfilaments 60 to define twist direction 66 (82). For example, the plurality of microfilaments 60 may be fed into a ring spinner or other fiber processing equipment. In some examples, continuous filament processing may be used to form continuous yarn structures. A variety of manufacturing parameters may be controlled to produce various parameters of shape memory yarn 42 including, but not limited to, feed rate of the plurality of microfilaments 60 into a spindle system and a rotational speed of the spindle system. For example, a relationship between a linear speed of the delivery and a rotational speed of the spindle may be used to achieve a desired amount of twist inserted into the yarn.

In some examples, the method of FIG. 3A may include torque balancing shape memory yarn 42 (84). For example, the insertion of twist of the plurality of microfilament 60 imposes a mixture of bending and torsional strains into the plurality of microfilaments 60. As a structure, this results in a torque imbalance that causes shape memory yarn 42 to untwist if both ends are not fixed. However, with both ends constrained, slack may cause kinking or pig tailing of shape memory yarn 42. In some examples, torque balancing shape memory yarns 42 may include thermal processing shape memory yarn 42 after spinning to shape-set the plurality of microfilaments 60 in their twisted structures and removes any residual stresses from the imposed manufacturing strains. These shape-set, or in this case, twist-set shape memory yarns 42 may continue to maintain structure upon slack. In some examples, a single shape memory yarn 42 may be plied together by twisting together two or more torque imbalanced single shape memory yarns in an opposite direction of an initial twist. In some examples, shape memory yarn 42 may not be torque balanced to remain an energetic torque imbalanced shape memory yarn, such as to provide additional actuation or customization of yarn performance.

In some examples, shape memory yarns 42 may be further process to form shape memory coils 40. The method of FIG. 3A includes forming a coiled shape memory yarn from shape memory yarn 42, such as coil portion 44 of FIG. 2A (90). Forming the coiled shape memory yarn may include coiling shape memory yarn 42 in coil direction 58 around coil axis 56 (92). The method of FIG. 3A may include torque balancing coiled shape memory yarn 42 (94). In some examples, forming shape memory coil 40 from shape memory yarn 42 and torque balancing shape memory yarn 42 may depend on twist direction 66 of shape memory yarn 42, as will be described further in FIGS. 3B and 3C below.

FIG. 3B is a flowchart of an example method of forming a homochiral shape memory coil in which coil direction 58 of shape memory yarn 42 and twist direction 66A of plurality of microfilaments 60 are the same. The method of FIG. 3B includes applying torsion in twist direction 66A of shape memory yarn 42 to form coils in shape memory yarn 42 (92A). This applied torsion may create a torsional imbalance in shape memory yarn 42. Once the torsional imbalance reaches a threshold, coils may form in shape memory yarn 42 in coil direction 58. The method of FIG. 3B may further include plying a plurality of coiled shape memory yarns to torque balance a plurality of shape memory yarns (94A). For example, a single coiled shape memory yarn 42 may be plied together by twisting together two or more torque imbalanced single coiled shape memory yarns in an opposite direction of twist direction 66A and coil direction 58.

FIG. 3C is a flowchart of an example method of forming a heterochiral shape memory coil in which coil direction 58 of shape memory yarn 42 and twist direction 66B of plurality of microfilaments 60 are different. The method of FIG. 3C includes wrapping shape memory yarn 42 around a mandrel to define coil direction 58, opposite to twist direction 66B, and form coils in shape memory yarn 42 (92B). The method of FIG. 3C may further include heat treating coiled shape memory yarn 42 to torque balance shape memory yarn 42.

Referring back to FIG. 3A, the method of FIG. 3A may include forming textile 12 from the shape memory coils 40 or shape memory yarns 42 (100). Forming textile 12 may include interlocking shape memory coils 40 and/or shape memory yarns 42 in one or more directions (102). Textile 12 may be formed through a variety of processes including, but not limited to, knitting, and the like. In some examples, forming textile 12 may include knitting shape memory coils 40 or shape memory yarns 42 into interlacing loops. Monofilament shape memory coils or wires that are not formed from shape memory yarns may have a high flexural rigidity, such that a knitting process may be difficult to control and may offer limited densities and drapabilities. In contrast, shape memory yarns 42 and/or shape memory coils 40 may retain sufficient strength with reduced flexural rigidity, such that shape memory coils 40 and/or shape memory yarns 42 may be particularly suited for relatively tortuous knitting processes. In some examples, the method of FIG. 3A may include heat treating textile 12 to shape set textile 12 (104).

Examples

NiTi Microfilament Yarns

Creation of micrometer-sized (e.g., <10 μm) NiTi filaments and their integration into yarn and textile structures may improve the tailorability of the mechanical performance of the NiTi material for diverse applications. Improvements may include additional and/or more easily controllable geometric parameters—yarn count, surface twist angle, manufacturing strains—that have been experimentally demonstrated to have impacts on NiTi mechanical performance such as a tunable structural stiffness, plateau strength, and cyclical consistency. Since flexural rigidity is important for textile manufacturing, the significant reduction in NiTi microfilament spun yarn flexural rigidity allows it to travel through traditional textile manufacturing equipment smoothly without the need for specialty monofilament feeders, thereby eliminating limitations seen in monofilament textile processing. Additionally, the inherent flexibility of a spun yarn may increase a wearability, tailorability, manufacturability, and lifetime performance of wearable devices. Textiles knitted from NiTi microfilament spun yarns exhibited both the superelastic and shape memory effects from a recovery of strains induced in the yarn and textile manufacturing processes.

Superelastic Mechanical Testing

For testing on an Instron, a sample was cut to a length of ≈100 mm and placed in pneumatic grips at pressures between 60 and 80 pounds per square inch (psi) at room temperature (23° C.). An environmental chamber was heated to 80° C. to heat the sample to austenite and remove residual strains from handling. At this point, any yarn slack was removed, and onset of tension was found at a 0.2 N applied load. A length of the yarn at this point was recorded as the austenite free length and was considered as the original length measurement for structural strain calculations. The sample was then displaced in tension at a constant rate at isothermal conditions while force was measured. The sample was then unloaded at the same constant displacement rate to back to a tension of 0.2 N. For shakedown testing, a similar loading and unloading path was cycled. For temperature-dependent testing, after unloading, the environmental chamber was then set to a new temperature while the sample was held at a constant force.

Shape Memory Mechanical Testing

Shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA). A similar procedure to superelastic yarns was completed to define the austenite free length in the DMA. A sample was cut to a length of ≈25 mm, placed in an environmental chamber, and heated to 80° C. Any yarn slack was removed, and an onset of tension was found at 10 grams (0.1 N) applied load. The length was then recorded and considered as the original length measurement for structural strain calculations.

Force generation is an important metric calculated from results from force blocking testing. For force blocking testing, the sample was loaded to and held at 2% structural strain while the environmental temperature was cooled to −40° C., soaked for 4 minutes, heated back to 80° C., and soaked there for 4 minutes. This cycle was repeated for a second time and upon completion, the sample was loaded to 4% structural strain. The same procedure was repeated at 6% structural strain and 8% structural strain.

Actuation contraction is an important metric calculated from results from free displacement test. A similar methodology was performed for free displacement testing as force block testing described above. The sample was loaded to 5 N and held while the environmental temperature was cooled to −40° C., soaked for 4 min, heated back to 80° C. and soaked for 4 min. This cycle was repeated twice and at 10, 15, and 20 N forces. For both experimental methodologies, the temperature ramps were programmed at 15° C. min⁻¹.

Microfilament Manufacturing

An accumulative drawing/rolling and bonding technique was used to create NiTi filaments with diameters in the micrometer range. The microfilaments were produced by drawing NiTi inside a sacrificial matrix by a conventional wire drawing process. FIG. 4A is an SEM image of 49 clusters of 31 NiTi microfilaments with diameters of 5 μm in a sacrificial iron matrix. The filaments were organized in clusters of individually nested filaments. FIG. 4B is an SEM image of two clusters of 31 NiTi microfilaments encased in an iron matrix. The bundle size is dictated by the number of clusters and the number of filaments in each cluster. 49 clusters containing 31 filaments each were drawn to form bundles of 1519 filaments with diameters of 5 μm (1519-5) and a total active cross-sectional area (Ay) of 2.98×10⁻² mm². The active cross-sectional area is a reference area based on the summation of individual microfilament cross-sectional areas. The matrix material was chemically removed, revealing the NiTi microfilaments. FIG. 4C is an SEM image of 19 NiTi microfilaments 5 μm diameters after etching of an iron matrix. Scanning electron microscopy (SEM) imaging of a single filament revealed drawing lines as well as occasional residual iron particles. FIG. 4D is an SEM image of a surface of a single 5 μm diameter NiTi microfilament with residual draw lines and iron particles. The NiTi alloy used has a chemical composition of 56 wt. % nickel with 300 ppm oxygen, 310 ppm carbon, and balanced titanium. It is a nickel-rich material, with an ingot AF of 68° C.

Digital scanning calorimetry (DSC) of pre-thermally processed microfilament bundles displayed unstable transformations (multiple austenitic transformation peaks and broad transformation ranges of 24° C.) and extreme transformation temperatures (martensite start temperature, MS=−53.9° C.; martensite finish temperature, MF=undefined). An in-line heat treatment apparatus for thermal processing of the NiTi bundles was manufactured to stabilize transformations and increase the martensite finish temperature. Bundles of microfilaments were heated in air at 500° C. for 120 seconds under 50 grams (0.5 N) of tension as determined by a design of experiments on the 1519-5 bundles. Post-thermally processed microfilament bundles demonstrated stabilized transformation temperatures (austenite finish, AF=47.2° C.; austenite start, AS=40.5° C.; martensite start, MS=−20.2° C., martensite finish MF=−29.8° C.) that were compatible with the temperature ranges of the environmental chamber for mechanical characterization.

The in-line heat treatment did not compromise the mechanical strength of the NiTi microfilaments. A 400-10 NiTi bundle was given a small producer's twist of 0.352 turns per centimeter (TPCM) to hold the structure together and mechanically tested at 80° C., above AF, to determine mechanical properties in the austenitic condition. For mechanical testing on the yarns, the active cross-sectional area was used to define a normalized force, which can be compared to stress in monofilaments. The filament bundles had an active cross-sectional area of 3.14×10⁻² mm², which was used to calculate the linear stiffness of the filament as 73.6 GPa before a stress-induced martensite (SIM) transformation occurred at a normalized force of 646.5 MPa (20.3 N). The filaments were able to achieve a loading plateau strain of 9.04% before restiffening occurred at a normalized force of 697.5 MPa (21.9 N). The filament bundle active cross-sectional area is approximately equivalent to the cross-sectional area of a 0.203 mm diameter monofilament wire. For benchmark comparisons, isothermal displacement-controlled testing of a 0.203 mm diameter wire of the same NiTi composition and thermal processing parameters was completed. The stiffness of the wire was 73.8 GPa before a SIM transformation at 519.1 MPa (16.8 N). The wire was able to achieve a loading plateau strain of 7.36% before restiffening at 522.2 MPa (16.9 N). The difference in strength between the filaments and benchmark wire may be explained by a difference in melts, processing history, the surface-to-volume ratio, and fewer defects in the microfilaments.

Yarn and Textile Manufacturing

The NiTi microfilaments were then processed through a hierarchical procedure from the straight bundles described in FIGS. 4A-4D above to plied twisted yarns to a knitted structure, as demonstrated in FIGS. 5A-5G.

FIG. 5A is a cross-sectional optical image at 20× magnification of a NiTi microfilament bundle before twist insertion. For NiTi yarn manufacturing, NiTi bundles were fed straight into an industrial ring spinner at a fiber processing and yarn spinning mill. FIG. 5B is a diagram of a yarn spinning process of a bundle of untwisted filaments through the delivery and spindle system of an industrial ring spinner into a twist inserted single yarn. In continuous filament (CF) yarn manufacturing, infinitely long filament bundles are twisted to form yarn structures. Industrial ring spinners may include two feed systems—a delivery system and a spindle system—to merge filament bundles into twisted yarns. The delivery system controls the feed rate of the filament bundles into the spindle system, which maintains a uniform tension while inserting a twist into the filaments. The relationship between the linear speed of the delivery system and the rotational speed of the spindle system can be used to estimate the total amount of twist (T) inserted into the yarn. Twist is one of three important material, geometric, and manufacturing parameters—yarn count (C), yarn twist (T), and packing factor (φ)—that are used to fully describe the geometry of the yarn in terms of the surface twist angle (a). Yarn count (C), also known as linear density, can offer insight into the amount of active material in a yarn. The final parameter, packing factor (φ), is the ratio of the specific volume of the filaments (v_(f)) to the specific volume of the yarn (v_(y)). The packing factor describes the efficiency of the filaments to pack within the yarn geometry. In these examples, manufacturing twist (T) was the main controllable parameter used to define the surface twist angle (a), which more accurately describes the final form of the yarn and can offer insight into the manufacturing strains and mechanical performance of the yarn.

When twisted, the individual NiTi microfilaments adapted successfully to this traditional yarn manufacturing method, and the microfilaments reconfigured into helixes. FIG. 5C is a SEM image of a twist inserted NiTi microfilament single yarn demonstrating the helical pathways of the continuous filaments on the surface of the yarn geometry. An imaging method can be used to estimate surface twist angle and yarn diameters. In an idealized helical structure, each filament follows a uniform helical path with a constant radius from the yarn axis and constant helical angle. The surface twist angle (α) and packing factor (φ) were measured using SEM and optical imaging to validate the NiTi yarn manufacturing parameters against traditional models. The NiTi material in the 1519-5 bundles had a density of 6.45 g cm⁻³, resulting in a yarn count of 192 tex. Tex is a yarn industry unit equivalent to grams per 1000 meters of yarn. A 192 tex yarn bundle was given a twist of 7.25 TPCM, which correlated to a packing factor of 0.530 and resulted in an estimated surface twist angle of 31.2°, which agreed with the measured surface twist angle of 30.4°.

Yarns with the same geometry can create stable torque balanced yarns or energetic torque imbalanced yarns. The insertion of twist imposes a mixture of bending and torsional strains into the microfilaments. As a structure, this results in a torque imbalance that causes the yarn to untwist if both ends are not fixed. With both ends constrained, any slack would result in a kinking or pig tailing of the yarn. Torque balanced yarns can be obtained using two approaches. In one approach, thermal processing of the yarns after spinning shape-sets the filaments in their twisted structures and removes any residual stresses from the imposed manufacturing strains. These shape-set, or in this case, twist-set, yarns maintain structure upon slack.

Alternatively, in another approach, the single yarns could be plied together by taking two or more torque imbalanced single yarns and twisting them together in the opposite direction of the initial twist. FIG. 5D is a diagram of a yarn plying process of multiple single twisted yarns spun together with an opposite twist direction to create a robust, torque stable structure. From a manufacturing approach, torque balancing the yarns was important for easier handling of the yarns for the knitting process. However, the energetic torque imbalanced yarns could provide additional actuation or customization of yarn performance. FIG. 5E is a SEM image of a NiTi microfilament two-ply yarn structure in a bent loop orientation to highlight an increased bending compliance afforded by the yarn structure.

Knitting is a manufacturing process in which long filaments or yarns are manipulated into interlacing loops. FIG. 5F is a diagram of a knit manufacturing process of a yarn structure being fed through a series of needles capable of manipulating the yarn into interlacing loops. The high flexural rigidity of monofilament NiTi wires make the knitting process difficult to control and offer limited densities and drapabilities. The retention of strength and decrease of flexural rigidity provide yarns with an ideal structure to survive the torturous knitting process. For example, a 0.203 mm diameter monofilament NiTi wire has a flexural rigidity of 5.84 N mm², compared to a flexural rigidity of 3.30×10⁻⁵ N mm² of a 1519 filament-5 μm diameter yarn, or a five orders of magnitude change in flexural rigidity for the same amount of active material. The NiTi yarns, with their decreased flexural rigidity, adapted well to the knitting process and exhibited the potential of NiTi yarns within a cloth-like textile. FIG. 5G is a SEM image of a section of a knitted textile manufactured with NiTi microfilament yarn.

Superelastic Performance of NiTi Microfilament Yarns

The impact of a variety of material, structural, and experimental properties of yarns on superelastic mechanical performance was investigated through a series of isothermal experiments, as illustrated in FIGS. 6A-6F. Experiments consisted of uniaxial tensile tests on an Instron 3365 in an Instron Environmental Chamber using pneumatic clamps to grip the yarn prototypes. Isothermal, displacement-controlled experiments used global displacement rates between δL⁻¹=5×10⁻⁵ s⁻¹ and δL⁻¹=4×10⁻⁴ s⁻¹. The slow displacement rates were necessary to dissipate heat during phase transitions to maintain isothermal conditions during testing.

Experiments were performed through a descending sweep of temperatures from above A_(F) to below M_(F), as found from DSC results. FIG. 6A is a graph of isothermal uniaxial testing results through a descending sweep of temperatures to demonstrate the thermal dependence on the mechanical behavior of NiTi microfilament yarns and for a preliminary understanding of the effects of yarn structure on typical NiTi behavior. The environmental temperatures, 80° C. (I), 40° C. (II), 0° C. (III), and −40° C. (IV) are in reference to the material transformational temperatures found through digital scanning calorimetry results. Before each loading, the 1519-5 3.26 TPCM torque balanced single yarn prototype was heated to 80° C., which is above AF, to recover any residual strains. At an experimental temperature of 80° C., the NiTi yarns displayed traditional austenitic and super-elastic behavior, starting with a linear stiffness of 43.7 GPa and an elevated SIM transformation onset of 543.6 MPa (16.2 N). The SIM transformation concluded at 8.19% structural strain at a normalized force of 583.9 MPa (17.4 N), corresponding with a restiffening of the material. The yarns were loaded to 9.23% structural strain. During unloading, the yarns exhibited a superelastic recovery of 8.43%, leaving 0.80% structural strain of unrecoverable deformations.

As the temperature decreases to 40° C., below AF but well above MS, the NiTi yarns exhibited a traditional NiTi thermal-mechanical dependency and thermally driven strain recoverability. During loading, the linear stiffness decreased to 25.4 GPa, the SIM transformation onset occurred at a lowered normalized force of 229.9 MPa (6.85 N), and restiffening occurred at an earlier structural strain of 6.69%. Although the yarns were initially austenitic and at a temperature above M_(S), the decreased temperature permitted earlier SIM transformation by decreasing the amount of energy needed to initiate the solid-state transformation. Upon unloading from 9.89% structural strain, the yarns exhibited limited unloading recoverability, only 3.90% structural strain, but upon heating above AF, thermally driven recoverable strains of 5.20% were reached, demonstrating the shape memory potential.

As the temperature decreases to 0° C., well below AF, but right before MS, the yarns displayed increased mechanical deviations from austenitic behavior. The material experienced a SIM transformation at the onset of loading, decreasing the initial stiffness to 14.7 GPa, as well as the normalized plateau force to 117.8 MPa (3.51 N) and restiffening strain to 6.12% structural strain. Upon unloading from 9.90% structural strain, the yarn recovered 2.95% strain, before an additional thermally driven recovery of 5.90% from heating above AF. Finally, at a temperature of −40° C., below MF, the material demonstrated fully martensitic behavior with a martensitic stiffness of 6.57 GPa, before detwinning occurred at a force of 59.1 MPa (1.76 N). Upon unloading from 9.90%, unloading strains of 2.70% structural strain were recovered. The yarns exhibited traditional NiTi monofilament behavior—superelastic recovery, tunable thermal-mechanical dependent stiffnesses, and recoverable residual strains that allow NiTi to be used in a variety of applications.

The influence of the twist on mechanical performance was evident from isothermal strain-controlled results. Under small structural strains of less than 1%, yarns exhibited an initial realignment in which the yarn radially contracted, the filaments began interacting, and the entire yarn began to deform linearly, which resulted in a very low and continuously changing stiffness region. FIG. 6B is a graph of uniaxial testing results for three NiTi microfilament single yarns spun at different twist levels to demonstrate the inverse relationship of twist on the linear stiffness region of austenitic NiTi. FIG. 6C is a graph of uniaxial testing results for NiTi microfiber two-ply yarn variants with different levels of twist to highlight the impact of twist on the reduced NiTi SIM transformation strength during loading performance. Yarns with larger twist values exhibited more extended low stiffness regions as demonstrated by the 7.25 TPCM torque balanced single yarn (III) extending to 0.57% strain before linearizing compared to 0.23% strain of the 3.26 TPCM torque balanced single yarn (I), such as shown in FIG. 6B.

Once linearly deforming, the effective yarn stiffness was inversely dependent on the surface twist angle of the yarn, which demonstrated the increased tailorability of yarn structures, as shown in FIGS. 6B and 6C. A simple model estimated that the effective yarn stiffness (E_(y)) varies with the cos 2(α) from the material stiffness, which suggested the impact of surface twist angle on yarn stiffness is more prevalent at higher surface twist angles than lower which is validated in our experimental results as the 7.25 TPCM yarn (III) exhibits an austenitic stiffness of 20.9 GPa, compared with the 5.08 TPCM yarn (II) austenitic stiffness of 34.9 GPa, and the 3.26 TPCM austenitic (I) stiffness of 38.7 GPa. The results in FIG. 6B aligned well with the simple model, in which the decrease in linear stiffness compared to the material stiffness was more dramatic at large surface twist angles, suggesting that existing yarn models may help predict the linear behavior of NiTi microfilament yarns.

In traditional austenitic NiTi behavior, there is a transition from a linear stiffness to a horizontal force plateau. In isothermally loaded monofilament NiTi wire, the transition is abrupt as the material uniformly undergoes a solid-state phase transformation from austenite to detwinned martensite. Within a yarn structure, this transition occurs over a range of structural strains that is heavily influenced by the amount of twist in the yarn, as shown in FIG. 6C. Larger twist values correlate to more diversity in filament stress states (extension, bending, and torsion) and complexity among filament pathways. For this reason, individual filaments will reach the material strain to cause a SIM transformation at different structural strains. Conversely, yarns with lower twist values act more like parallel filaments and exhibit more uniform SIM transformations. Qualitatively, this can be observed in the abrupt change in the 3.26 TPCM two-ply yarn (I) at 1.79% structural strain compared to the gradual SIM onset in the 7.25 TPCM two-ply yarn (II) from ≈2.45% structural strain to 4.10% structural strain, as shown in FIG. 6C.

To add strength in monofilament wire applications, engineers typically choose larger wire diameters; however, this added strength comes with a significant increase in flexural rigidity. In the twisted yarn form, strength is increased by increasing the amount of active material in the yarn through plying, which adds significant strength to the yarn structure at a small increase to the flexural rigidity. FIG. 6D is a graph of uniaxial testing results comparing austenitic loading profiles of a NiTi microfilament single yarn to a similar spun NiTi microfilament two-ply yarn to demonstrate the increased strength capabilities of plied yarns. The 3.26 TPCM torque balanced single yarn (I) exhibited a plateau onset force of 16.7 N (560.4 MPa) and an estimated flexural rigidity of 6.40×10⁻⁴ N mm², while the 3.26 TPCM two-ply yarn (II) exhibited a plateau onset force nearly double that of the single yarn at 32.9 N (552.0 MPa), and an approximate doubling in flexural rigidity at 1.28×10⁻³ N mm². The act of plying removed some of the initial twist imposed on the single yarns resulting in an increase in linear stiffness in the plied yarn from 37.4 to 39.6 GPa. While plying may increase flexural rigidity, this increase may be minor. For example, in area equivalent monofilaments, a 0.203 mm diameter wire (1519-5 single yarn) would have a flexural rigidity of 5.84 N mm² compared to a 0.2794 mm diameter wire (1519-5 two-ply yarn) flexural rigidity of 20.9 N mm², an increase of 3.58 times. Additionally, plied yarns may still be easily manipulated and capable of being manufactured into a textile structure. Plying plays a significant role in the increased manufacturability and tailorability of yarn structure applications that require strong and compliant solutions.

Mechanically, there are many qualitative and quantitative differences between torque balanced yarns and torque imbalanced yarns as a result of the impact of thermal processing on the structure and filament strains within the NiTi yarn. FIG. 6E is a graph of uniaxial testing results comparing a torque balanced NiTi microfilament single yarn to a torque imbalanced NiTi microfilament single yarn to demonstrate the impact of manufacturing stresses on the reduced NiTi SIM transformation strength during mechanical loading. With their desire to untwist, filaments in torque imbalanced yarns attempt to radially separate from each other. One torque balanced (II) and one torque imbalanced (I) single yarn, each with 1519-5 filaments and 3.26 TPCM, were experimentally tested. Upon loading from the length at austenite zero force, the yarn underwent radial contraction and filament realignment, resulting in an exaggeration of the initial filament realignment region before linear stiffness. Once linear, the linear stiffness of the torque imbalanced yarn of 28.6 GPa was reduced compared to the linear stiffness of the torque balanced yarns of 44.8 GPa. The act of heat treatment under a light tension (50 grams) pulled the filaments closer together and shape-set them in that position, resulting in a reduction in twist. The force plateau onset normalized force of the torque balanced yarns was slightly less than the torque balanced yarns, 580.5 MPa (17.3N) compared with 634.2 MPa (18.9N), as a result of the remaining residual stresses from yarn manufacturing in the torque imbalanced yarns. Thermal processing permits designers to impact mechanical behavior beyond the manufacturing stage, expanding the variety of ways to tune performance.

Consistent mechanical performance is important in many NiTi applications. For the NiTi material system, material shake-down is an understood behavior, while structural shakedown is recognized to be prevalent in highly frictional structures, but not thoroughly explored. The mechanical behavior of NiTi yarns differs significantly between the first and all subsequent loading cycles. FIG. 6F is a graph of cycled loading results (I, II, III, IV, V, VI) for a NiTi microfilament 3.26 TPCM single yarn structure to show the effect of structural and material shakedown of NiTi microfilament yarns demonstrating a consistent loading and unloading path. Upon first loading of a 3.26 TPCM single yarn, the plateau onset normalized force was at 513.4 MPa (15.3 N), and although it was unloaded at the start of restiffening, there remained a 0.78% permanent strain after superelastic recovery. Upon reloading to the same force, the upper plateau onset normalized force saw a significant decrease to 409.4 MPa (12.2 N), and restiffening of the detwinned martensite occurred at earlier strains, 7.85% structural strain. Upon superelastic recovery, the structure demonstrated a similar unloading path to the first cycle; however, the decrease in upper plateau from the first to second cycle resulted in a reduction in the hysteresis loop—the mechanical branching between loading and unloading from a reversal in transformation direction. Once fully unloaded, there remained a decreased permanent strain of 0.312% structural strain compared to the first cycle. The linear stiffness saw a slight decrease from the first cycle value of 38.7 GPa compared to the last cycle value of 36.6 GPa. While some of the differences between the first and second cycles can be attributed to material shakedown, the difference can also be attributed to the filaments being pulled out of their original configurations into positions that, upon unloading, were not reversible. The filaments then remained in a higher deformed and stressed state, and upon reloading, detwinned at lower forces.

To differentiate material shakedown from structural shakedown, shakedown testing was conducted on the benchmark 0.203 mm wire. The difference between the yarn and wire shakedown testing was most evident between a large change in behavior between the first and second loading cycle in the yarn compared with the wire. This suggests that, upon first loading, the filaments in the yarn structure were undergoing a structural settling on top of material settling. All subsequent loading cycles of the NiTi yarn were similar but did exhibit minor decreases in plateau onset strength, unrecoverable deformations, and increases in plateau stiffness. However, with each subsequent cycle, these differences decreased, suggesting a consistent hysteretic loop and filament configuration may be attainable by further cycling of the NiTi yarn structure. It is important to note that the shakedown phenomenon described herein can also be seen in the temperature dependence test between the first cycle at 80° C. to subsequent cycles, as shown in FIG. 6A.

Shape Memory Effect in NiTi Microfilament Yarns

To demonstrate the shape memory ability of the NiTi microfilament yarns, free displacement (isoforce) and force blocking (isostrain) testing was performed on a 400-10 3.51 TPCM torque balanced single yarn and a 400-10 7.03 TPCM torque balanced single yarn. Shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA) with an attached environmental chamber. During testing, displacement-controlled portions were executed at global displacement rates between δL⁻¹=+5×10⁻⁵ s⁻¹ and δL⁻¹=+4×10⁻⁴ s⁻¹, while isostrain/isoforce temperature ramps were performed at 15° C. min⁻¹. At the end of each temperature ramp, the sample was soaked for 240 s to ensure the completion of the solid-state transformation.

In free displacement testing, an ability of the yarn to produce contractions at a prescribed force, called the actuation contraction, is experimentally assessed. In actuating systems that integrate artificial muscles, the actuation contraction is critical for designers to achieve desired motions under application-level loads. At 5.0 N applied force, the 7.03 TPCM torque balanced single yarn was able to cool to a longer martensitic length than the 3.51 TPCM torque balanced single yarn, and upon heating, was able to recover more of deformations; however, more unrecoverable deformations remained as a result of internal friction in the yarn. FIG. 7A is a strain-force graph illustrating a 3.51 TPCM torque balanced single yarn that recovered a substantial portion of deformations using the shape memory effect. FIG. 7B is a strain-force graph illustrating a 7.03 TPCM torque balanced single yarn that partially recovered deformations using the shape memory effect. At 10.0 N applied force, the 7.03 TPCM yarn was once again able to cool to a longer martensitic length than the 3.51 TPCM yarn, but the culmination of permanent deformations impacted the ability of this yarn to recover upon heating, resulting in more recoverable deformations in the 3.51 TPCM, as shown in FIGS. 7A and 7B. FIG. 7C is an actuation-force graph illustrating the 3.51 TPCM yarn that provided a maximum actuation contraction under a 10 N applied load, while the 7.03 TPCM yarn provided a maximum actuation contraction at 5 N. The 3.51 TPCM yarn had a maximum average actuation contraction of 4.38% at a force of 10.0 N over two thermal cycles compared to the 7.03 TPCM yarn, which had a maximum average actuation contraction of 3.46% at a force of 5.0 N over two thermal cycles.

While the maximum average contraction of the 3.51 TPCM was larger than that of the 7.03 TPCM yarn, the influence of twist enabled greater actuation displacements at lower forces, providing a design tradeoff in the shape memory performance. Despite maximum average actuation contractions occurring at different forces, both yarns produced average maximum specific works under a 10.0 N applied load where the 3.51 TPCM yarn produced 2338 J kg⁻¹ of specific work compared to the 7.03 TPCM yarn which produced 1480 J kg⁻¹ of specific work. The higher twisted yarn had more consistent actuation contractions across a larger range of forces, as observed by the 21.9% change in actuation contraction in the 7.03 TPCM yarn from 5.0 to 10.0 N, compared to the abrupt 84.2% change in actuation contraction in the 3.51 TPCM yarn from 5.0 to 10.0 N.

Conversely, in force blocking testing, the generated force of the yarn at a prescribed structural strain is the metric of interest. In compression garment applications, understanding the forces generated at a specific strain is the basis for accurately designing a garment capable of producing pressures that offer medical benefits to the wearer. The 3.51 TPCM torque balanced single yarn saw elevated forces in austenite when compared to the 7.03 TPCM torque balanced singe yarn as a result of increased stiffness and SIM transformational strength. FIG. 7D is a strain-force graph illustrating the 3.51 TPCM yarn that provided higher forces in both martensite and austenite when stretched by the same amount. FIG. 7E is a strain-force graph illustrating a 7.03 TPCM yarn. The elevated forces of the 3.51 TPCM were also present at martensitic lengths. FIG. 7F is an actuation-force graph illustrating that, despite differences in the absolute force in each yarn, both the 3.51 TPCM and 7.03 TPCM single yarn produced qualitatively and quantitatively similar generated forces during force blocking tests from 2% to 8%. Despite the difference in magnitude of the force on each twisted yarn, both the 3.51 TPCM and 7.03 TPCM yarns had similar generated force profiles. Over the structural strain range of 0% to 8%, both yarn variants saw a consistent increase in generated forces from ≈4.1 N at 2% structural strain to 12.75 N at 8% structural strain. Although there are many design tradeoffs with an increase of twist, the generated forces remain consistent across the tested yarn geometries.

Textile Performance

Knit manufacturing involves a new set of design parameters that impact the final textile form, including needle spacing, yarn feed tension, and knit pattern. The needle spacing determines the course width while the tension system, which controls the length of material in one loop, determines the wale height. The knit pattern, which is a collection of knit and purl loops, influences the mechanical behavior of the passive and active textiles. Heat treatment at this stage of manufacturing can also tune the material and structural behavior. To demonstrate the hierarchical process from start to finish, three NiTi bundles were given a twist of 4.37 TPCM before being plied together at a twist of 1.46 TPCM. A torque balanced three-ply NiTi yarn was knitted into a garter pattern textile, which has alternating rows of purl and knit loops, on a Taitexma TH-860 hobbyist knitting machine with a needle spacing of 4.5 mm and a tension setting of 8.

For NiTi monofilament wire, the knitting process induces bending deformations that promote large forces, unprecedented actuation contractions, and variable stiffness behavior. NiTi textiles composed of microfilaments were able to demonstrate similar shape memory, as illustrated in FIG. 8A-8D, and superelastic behavior, as illustrated in FIGS. 8E and 8F, to monofilament knits. In this case, a mixture of torsional and bending strains from both the yarn manufacturing and knitting processes enabled strain recovery through superelasticity and shape memory effect.

For a benchmark comparison, the three-ply garter knit was stretched and constrained at room temperature, 23° C., above MF and below AS, and then released. FIG. 8A is a photograph of a microfilament NiTi textile when loaded at room temperature. FIG. 8B is a photograph of the microfilament NiTi textile when released at room temperature. The knit slightly recovered from a combination of austenite remaining in the material at room temperature, and from an inherent knitted textile elasticity. To demonstrate the superelastic effect, the knitted textile was stretched on a hot plate at a temperature of 90° C., above AF. FIG. 8C is a photograph of the microfilament NiTi textile when loaded at temperatures above AF of the constituent NiTi microfilaments. Upon the release of the end constraint, the knit immediately sprung back into its pre-stretched state. FIG. 8D is a photograph of the microfilament NiTi textile when released at temperatures above AF of the constituent NiTi microfilaments to highlight the enhanced recoverability afforded by the superelastic material behavior. A significant difference in recoverable strain and recovery recoil the stretched state, 36% at room temperature compared to 65% with heating was observed. To demonstrate the shape memory effect, the same knit was placed on the hot plate and stretched and constrained at room temperature. FIG. 8E is a photograph of a microfilament NiTi textile when unloaded at room temperature. Once the length constraint was removed, the knit recovered slightly to the position described with respect to FIG. 8B. The hot plate was then activated to heat the knit to 90° C. above A_(F), in which the knit continued to contract substantially to near its pre-stretched state, an additional strain recovery of 20% relative to the austenitic unloaded state. FIG. 8F is a photograph of the microfilament NiTi when heated above AF to highlight actuation via the shape memory effect.

NiTi Microfilament Coiled Yarns

This work presents the novel creation of NiTi microfilament over-twisted coiled yarns and outlines the ideal applied stress at twist insertion to ensure successful coil propagation. Uniaxial testing and empirical analysis quantified the global actuation, force generation, cycled consistency, thermomechanical dependence, and superelastic performance of NiTi over-twisted coiled yarns. NiTi microfilament over-twisted coiled yarns offer exciting new compact solutions for energy absorbing and actuating applications in wearable, medical, and robotic industries.

Experimental Methodology

Both superelastic and shape memory mechanical testing was performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA) equipped with an environmental chamber for controlling temperature. To begin all testing, the chamber is heated to 80° C. to heat the sample to austenite and remove and residual strains from handling. At this point, any slack is removed, and the onset of tension is found at 10 grams applied load. The length at this point is recorded as the austenite free length and is considered as the original length measurement for structural strain calculations. Superelastic mechanical performance was investigated through a series of isothermal, displacement-controlled experiments which used global displacement rates between δ L⁻¹=±5×10⁻⁵ s⁻¹ and δL⁻¹=±4×10⁻⁴ s⁻¹. The slow displacement rates were necessary to dissipate heat during phase transitions to maintain isothermal conditions during testing.

Shape memory effect was investigated through two different testing methodologies, free displacement (isoforce), and force blocking (isostrain), such as described in Shape Memory Effect in NiTi Microfilament Yarns above. For free displacement testing, the sample was loaded to 2 N and held while the environmental temperature was cooled to −50° C. at a rate of 30° C./min, soaked for four minutes, heated back to 80° C. at the same rate, and soaked for four minutes. This cycle was repeated twice and at 4N, 6N, and 8N forces. The actuation contraction was the key metric calculated from the results from free displacement test and is defined as the difference in the martensitic length at the end of a cool ramp (l_(mar)) and the austenitic length at the end of the subsequent heating ramp (l_(aus)), divided by the martensitic length. For each force, there were two actuation contractions derived, and the average was used analytically. A similar methodology was performed for force blocking testing. The sample was loaded to 10% structural strain and held there while the environmental temperature is cooled to −50° C., soaked for four minutes, heated back to 80° C., and soaked there for four minutes. This cycle was repeated for a second time, and upon completion, the sample was loaded to 20% structural strain. The same procedure was performed at 30% structural strain. Force generation was the key metric calculated from the results from force blocking testing and is defined as the difference between the martensitic force (F_(mar)) at the end of a cooling ramp, and the austenitic force (F_(aus)) at the end of the subsequent heating ramp. For each strain, there were two force generations and the average was used analytically.

Microfilament Manufacturing

As described above, the creation of NiTi over-twisted yarns was enabled by the implementation of an accumulative drawing/rolling and bonding technique to create NiTi filaments with diameters in the micron-range. The microfilaments are produced by drawing inside a sacrificial matrix by a conventional wire drawing process. The filaments are organized in clusters of individually nested filaments. The matrix material is chemically removed, revealing the NiTi microfilaments. SEM imaging of a single filament reveals drawing lines as well as occasional residual iron particles.

The bundle dimensions are dictated by the number of filaments and diameter of the individual filaments. In these examples, bundles containing filaments with diameters of 10 μm in 400, 800, 1200, or 1600 increments were used. The active cross-sectional area is a reference area based on the summation of individual microfilament cross-sectional areas. For instance, a bundle of 400 filaments with diameters of 10 μm (400-10) has a total active cross-sectional area (A_(y)) of 3.02×10⁻² mm². The NiTi alloy used in this study has a chemical composition of 56 wt. % nickel with 300 ppm oxygen, 310 ppm carbon, and balanced titanium. The NiTi alloy was a nickel-rich material, with an ingot A_(F) of 68° C.

Digital scanning calorimetry (DSC) of pre-thermally processed microfilament bundles unstable transformations (multiple austenitic transformation peaks and broad transformation ranges of 24° C.) and extreme transformation temperatures (martensite start temperature, M_(S)=−53.9° C.; martensite finish temperature, M_(F)=undefined). A custom heat treatment apparatus for thermal processing of the NiTi over-twisted coiled yarns was manufactured to stabilize transformations and increase the martensite finish temperature. Over-twisted coiled yarns were heated in air at 550° C. for 11 minutes as determined by a design of experiments on the 1519-5 bundles. Post-thermally processed microfilament bundles demonstrated stabilized transformation temperatures (austenite finish, A_(F)=52.5° C.; austenite start, A_(S)=40.8° C.; martensite start, M_(S)=−10.3° C., martensite finish M_(F)=−23.2° C.) that were compatible with the temperature ranges of the environmental chamber for mechanical characterization.

Over-Twisted Coiled Yarn Manufacturing

Over-twisted coiled yarns may be manufactured using industrial manufacturing equipment. FIG. 9A is a diagram of an over-twisted coil yarn manufacturing system equipped with a DC motor, a rotary encoder, a pulley system for force loading, and a linear encoder. A custom-built coil manufacturing rig inserts twist into a microfilament bundle at an applied tension load to manufacture over twisted coiled yarns. One end of a strand of a microfilament bundle is clamped directly into a DC motor shaft, while the other is fixed on a custom 3D printed platform that restricts any further twisting, causing all inserted twist to act on the microfilament strand. The custom 3D platform is connected by a pulley system to an applied weight to control the tension in the strand. During manufacturing, tracking of the inserted twist and strand length is done by a rotary encoder on the DC motor and a linear encoder strip attached along the pulley system. At the start of coil propagation, the length and twist are recorded to divide the manufacturing process into pre-coil (yarn angle progression) and post-coil sections (coil propagation).

Various coiled forms may be created by controlling an amount of stress to NiTi bundles. FIG. 9B is a diagram illustrating four main manufacturing results with a direct relationship to the amount of stress applied to the NiTi bundles. When a bundle of filaments is twisted, a torsional imbalance is created in the yarn resulting in spontaneous coil forming in two directions. At low or zero tension, the yarn reconfigures in a torque balanced snarl (or pigtail) in a direction normal to the yarn axis. As you increase the tension, axial homochiral cylindrical coil will form in multiple locations, however this can lead to defects in the coiled yarn (double coiling, uncoiled sections), that compromise the mechanical integrity of the structure. At ideal tension, the yarn will form an axial homochiral cylindrical coil at a single location and propagate from there. At high tension, the yarn will break before any snarling occurs.

Thermal processing is required to shape set the NiTi filaments and remove the residual torsional stresses that remain in the coiled yarn structure after manufacturing. During thermal processing, the coiled yarn was fixed in place after manufacturing and heat treated in a Lindberg/Blue M™ boxfurnace at 550° C. for 11 minutes. As well as torsional stability, post-thermally processed coiled yarns demonstrated stabilized transformations that were compatible with the temperature ranges of the environmental chamber for mechanical characterization. FIG. 9C is a graph of DSC results for pre-processed (I) and post-processed (II) NiTi material demonstrating enhanced transformational stability and defined transformational regions.

FIG. 9D is a SEM image of 800-10 over-twisted coiled yarn. Post-thermally processed microfilament bundles demonstrated stabilized transformation temperatures (austenite finish, A_(F)=52.2° C.; austenite start, A_(S)=40.7° C.; martensite start, M_(S)=−10.3° C., martensite finish M_(F)=−22.7° C.). Additionally, thermal processing was performed to shape-set the NiTi filaments and remove residual torsional stresses that remained in the OTC yarn structure after manufacturing. After processing, the NiTi microfilaments demonstrated structural integrity via torsional stability in axially configured homochiral over-twisted coiled yarn structures.

The over-twisted coiled yarn may form a basis for more complex one-dimensional structures, such as NiTi artificial muscle architectures. Additional OTC yarn structures were manufactured by manipulating an OTC yarn after manufacturing but before thermal processing and shape setting. The different artificial muscle architectures illustrated in FIGS. 9E-9G may exhibit tailorable behavior through structural leveraging, such as increased structural strains, large actuation displacements, torsional stability, and scalable generated forces.

FIG. 9E is a SEM image of a plied 800-10 over-twisted coiled yarn. The plied OTC yarn of FIG. 9E was manufactured by pinching an OTC yarn in the middle, hanging a weight off both free ends, and allowing the structure to correct any torsional imbalance through self-plying. The act of plying enables scaling of the amount of active material without alterations to the bundle configuration. Additionally, plying may provide torsional stability achieved independently from thermal processing, offering designers more behavioral tunability from the inclusion of inserted twist manufacturing stresses within the stress-dependent SMA material system.

FIG. 9F is a SEM image of an 800-10 over-twisted coiled yarn spring. The OTC yarn spring of FIG. 9F was manufactured by tightly wrapping an OTC yarn around a thin mandrel wire (0.1 mm diameter). For artificial muscle applications that require large structural strains (>100%), the mandrel-wrapped spring structure can be used. The OTC yarn spring demonstrated a similar ability of increased structural strains from additional torsional geometric leveraging of the spring structure.

FIG. 9G is a SEM image of a loop made from 400-10 over-twisted coiled yarn. An OTC yarn structure may be used as a 1D basis of a hierarchical knitted textile in which the OTC yarns are manipulated into repeating interlocking loops, as illustrated in FIG. 9G.

Manufacturing Study

A manufacturing study was performed to identify ideal tensions for different bundle configurations to ensure successful coil propagation and offer insight into coil manufacturing mechanics. FIG. 10 is a graph summarizing the manufacturing study to determine ideal applied stresses to the NiTi microfilaments to ensure successful coil propagation. NiTi microfilament bundles consisting of 400, 800, 1200, and 1600 filaments were twisted at various levels of applied tension and categorized with one of the four results depicted in FIG. 9B. For the 400-10 configuration, which had an Ay of 3.02×10⁻² mm², the ideal applied stress range was from 13.9 MPa to 18.6 MPa. The 800-10 bundle had an Ay of 6.04×10⁻² mm² and an ideal applied stress range of 8.50 MPa to 15.0 MPa. The 1200-10 had an Ay of 9.06×10⁻² mm² and an ideal applied stress range of 10.9 MPa to 15.8 MPa. Lastly, the 1600-10 bundle had an Ay of 6.04×10⁻² mm² and an ideal applied stress range of 11.4 MPa to 12.5 MPa. The variation in stress ranges was due to inconsistencies (e.g., over-etching, breakage) along the NiTi microfilament bundle. However, the results were useful for guiding future manufacturing to limit material waste as well as to minimize structural variation during mechanical characterization.

During the manufacturing study, the sample length, inserted rotations, and coil initiation were recorded for each sample. To normalize across sample sizes, the sample length was normalized into a retraction. The retraction is defined as a difference between length of zero-twist sample (l_o) and twisted sample (l), divided by an original length of the zero-twist sample. The inserted rotations were normalized into the twist per unit length by dividing the rotations by the zero-twist length. During the twist angle progression stage (pre-coil) of coiled yarn manufacturing, imaging of the yarn can be used to estimate the geometry of the filaments in the yarn. In an ideal yarn theory, when a bundle of filaments is given twist, the filaments reconfigure in helical pathways each sharing the same pitch height defined as the inverse of inserted twist. Each helix has a constant radius (r) from the yarn axis and helical angle (θ) defined as the acute angle between the yarn axis and filament axis. The helical angle ranges from 0 at the yarn core, to the surface twist angle at the outside of the yarn which can be estimated from the yarn retraction (R).

FIGS. 11A-11D illustrate a graphical progression of sample retraction, and yarn bias angle increase as a result of twist insertion during the manufacturing process of over-twisted coiled yarns. FIG. 11A demonstrates a typical curve of retraction (I) and yarn angle bias (II) for a 400-10 bundle configuration during coil manufacturing. FIG. 11B demonstrates a typical curve of retraction (I) and yarn angle bias (II) for an 800-10 bundle during coil manufacturing. FIG. 11C demonstrates a typical curve of retraction (I) and yarn angle bias (II) for a 1200-10 bundle during coil manufacturing. FIG. 11D demonstrates a typical curve of retraction (I) and yarn angle bias (II) for a 1600-10 bundle during coil manufacturing. Each plot is divided into a pre-coil stage where the yarn bias angle sees a steady increase, and a post-coil stage where the yarn angle estimation is no longer valid. In each plot, there is a clear distinction in retraction behavior between the two stages. Additionally, with increasing filament counts, there is a decrease in the amount of twist needed to induce a coil. Across the bundle configurations, the yarn angle relationship remains consistent, with the first coil occurring in to 52.4°-61.5° range. However, the amount of twist needed to achieve coil initiation decreased as the number of filaments in each bundle increased. Additionally, across the bundle configurations, the retraction at first coil initiation remained consistent in the 0.65 to 0.76 range; however, as the number of filaments increased, the amount of inserted twist to achieve this retraction decreased. Visually, there was a clear change in retraction behavior from a nonlinear state in pre-coiling to a linearized relationship in post-coiling.

Over-Twisted Coiled Yarn Characterization

After manufacturing, the over-twisted coiled yarns made from different bundle configurations were geometrically characterized in a scanning electron microscope (SEM) to identify the coil diameter (D_(c)), coil angle (α_(c)), and yarn diameter (D_(y)), such as coil diameter 52, coil angle 51, and yarn diameter 48 of FIG. 2A. The coil diameter is defined as the radial length of the global coil structure, while the yarn diameter is defined as the radial length of the yarn constituting a single coil. The coil angle describes the acute angle between the coil axis and yarn axis. FIG. 12A is a SEM image of 800-10 over-twisted coiled yarn to define the geometric parameters used to describe the final form of the coiled yarn. FIG. 12B is a graph of average geometric parameter values measured from SEM images on four bundle configurations with increasing amounts of filaments. The geometric definitions can be seen in FIG. 12A, while the average dimensions for the 400, 800, 1200, and 1600 filament bundles used in the manufacturing study can be seen in FIG. 12B. There appeared to be little relationship between the number of filaments and the coil angle; however, there was a strong positive correlation between the number of filaments and yarn and coil diameters.

Superelastic Performance of NiTi Over-Twisted Coiled Yarns

Preliminary superelastic testing was interested in determining structural strain capabilities as well as safe operating ranges of the over-twisted coiled yarn structure. An 800-10 over-twisted coiled yarn was mechanically tested to increasing structural strains in 5% increments at 80° C., above A_(F). FIG. 13A is a graph of preliminary isothermal uniaxial displacement-controlled test results on an 800-10 coiled yarn to increasing structural strains. The coiled yarn was able to displace and unload from 40% structural strain before failing attempting to reach 45% structural strain. However, this extended structural strain range came as a tradeoff in the force capabilities, as the coiled yarn remained under 10 N applied load. Additionally, displacing the coiled yarns to large structural strains resulted in significant unrecoverable deformations seen in the difference in structural strain at the beginning of the loading cycle to the end of the unloading cycle. The unrecoverable strains are at a minimum of 0.359% during the 15% loading cycle compared to a maximum of 2.45% during the 40% loading cycle. To limit the number of unrecoverable deformations, the rest of superelastic testing was determined to have maximum displacements of 20% structural strain.

In many NiTi applications, the thermomechanical coupling acts as the mechanism for actuation and force generation. To determine the extent of thermomechanical coupling in NiTi over-twisted coiled yarns, isothermal testing was performed at 80° C., above A_(F) and −50° C., below M_(F), as found from DSC results illustrated in FIG. 9C. FIG. 13B is a graph of uniaxial testing on an 800-10 coiled yarn at temperatures above A_(F) and below M_(F) to demonstrate the thermomechanical coupling of NiTi coiled yarns. Before each loading the 800-10 coiled yarn prototype was heated to 80° C., which is above A_(F), to recover any residual strains. At an experimental temperature of 80° C., the NiTi yarns display aspects of traditional austenitic and superelastic behavior, initially loading linearly with a stiffness of 1.15 N·Structural Strain⁻¹ and a structural modulus of 2.31 MPa. At approximately 3% structural strain, the OTC yarn behavior begins to differentiate due to the onset of material phase transformation from austenite (I) to a stressed induced de-twinned martensite (II) (A→M^(d)).

In traditional austenitic NiTi behavior, there is a transition from a linear stiffness to a horizontal force plateau. In isothermally loaded monofilament NiTi wire, the transition from linear loading to a constant force plateau is abrupt as the material uniformly undergoes a solid-state phase transformation from austenite to stress induced martensite (SIM). Within a coiled yarn structure, this transition occurs over a range of structural strains that is due to more diversity in filament stress states (extension, bending, and torsion) and complexity among filament pathways. For this reason, individual filaments will reach the material strain to cause a SIM transformation at different structural strains. In this case, this austenitic OTC yarn exhibits A→M^(d) transformation from approximately 3.0% to 12.0% structural strain. During unloading, the OTC yarn exhibits a reverse transformation (M^(d)→A) back to austenite, resulting in superelastic recovery of 18.45% structural strain and 1.55% of unrecoverable structural strain. Superelastic SMA monofilaments exhibit large hysteresis between the austenitic loading and unloading curves. The potential of these coils as energy absorbers is highlighted in this area between the austenitic loading and unloading curve (84.2 N·Structural Strain) as mechanical energy is absorbed and dissipated as heat during material transformation to stress induced martensite. This area represents mechanical energy that is absorbed and dissipated as heat during the forward and reverse SIM transformations. Hysteresis also occurred in the complex OTC yarn structure, resulting in 1670 kJ m⁻³ of dissipated energy per unit volume. The dissipated energy offers insight into the multifunctional capabilities of over-twisted coiled yarns as compact energy absorbers and elastocaloric coolers that are not observable in other material systems.

At an experimental temperature of −40° C. (Martensite, FIG. 14B), the OTC yarn exhibits an initial twinned martensitic modulus of 1.02 MPa, or a 2.26 times decrease in stiffness from austenite. At this temperature, the filaments transform from self-accommodating twinned martensite to de-twinned SIM from approximately 3.0% to 11.0% structural strain (M^(t)→M^(d)). The M^(t)→M^(d) transformation occurs at significantly lower forces (˜1.5N to ˜3.6N) compared with the A→M^(d) transformation (˜3.3 N to ˜8.0N). Upon unloading, the OTC yarn exhibited a non-linear recovery of 7.53% structural strain. In traditional monofilaments, martensitic unloading behavior occurs linearly with limited recovered strain. Increased martensitic strain recovery in the OTC yarns indicates the OTC yarn structure plays a role in recoverability due to variance among filament stress states. Analysis of isothermal loading of the OTC yarn structures and comparison between austenite and martensite provides theoretical actuation boundaries, insight into structural effects, and an understanding of SIM transformation within the new form factor.

Consistent mechanical performance is important in many NiTi applications. For the NiTi material system, material shakedown is an understood behavior, while structural shakedown is recognized to be prevalent in highly frictional structures, but not thoroughly explored. The mechanical behavior of NiTi coiled yarns differs significantly between the first and all subsequent loading cycles. Upon first loading of the 800-10 coiled yarn, the onset and end forces of SIM transformation was estimated to be 3.48N and 8.70N, respectively. After unloading from 20% structural strain, there remained a 1.55% permanent strain after superelastic recovery. Upon reloading to the same strain, the linear stiffness decreased from 1.15 N·Structural Strain⁻¹ to 0.95 N Structural Strain⁻¹, while the onset and end of SIM transformation was estimated to be 3.11N and 6.55N, a large decrease from the first cycle. Upon superelastic recovery, the structure demonstrated a similar unloading path to the first cycle; however, the decrease in SIM transformation from the first to second cycle resulted in a reduction in the hysteresis loop—the mechanical branching between loading and unloading from a reversal in transformation direction. Once fully unloaded, there remained a decreased permanent strain of 0.95% structural strain compared to the first cycle.

FIG. 13C is a graph of cycled uniaxial testing of an 800-10 coiled yarn to demonstrate material and structural shakedown and the ability for the structure to reach a consistent loading path. Further loading cycles of the coiled yarn indicate a consistent loading/unloading pathway is achievable as unrecoverable deformations decreased from a first cycle (I) to 0.19% structural strain by the 10th cycle (II). Consistent cycled behavior (10th cycle) demonstrated an initial stiffness 0.75 N Structural Strain⁻¹ and hysteresis of 21.8 N Structural Strain.

While some of the differences between the first and subsequent cycles can be attributed to material shakedown, the difference can also be attributed to the micro filaments being pulled out of their original configurations into positions that, upon unloading, are not reversible. The filaments then remain in a higher deformed and stressed state, and upon reloading, will detwin at lower forces. Additionally, due to their complex pathways, some microfilaments may be plastically deforming at large structural strains. If enough microfilaments are plastically damaged, their individual superelastic recoveries may be limited and the global response of the coiled yarn may exhibit large unrecoverable deformations.

Shape Memory Effect Actuation Performance of NiTi Over-Twisted Coiled Yarns

For more dynamic insight into the shape memory potential of over-twisted coiled yarns, free displacement and force blocking testing was performed. In free displacement testing, the impact of internal friction within a system can be assessed by studying the recoverability of the sample during thermal cycling, such as by quantifying the contraction actuation potential of the OTC yarn at various constant applied forces.

FIG. 14A is a graph of full structural strain and force curves for free displacement testing of an 800-10 coiled yarn. For the 800-10 over-twisted coiled yarn, large changes in length were exhibited during the first cooling ramp, but limited recoverability was demonstrated upon heating. For example, during the first cooling from 2N, the coiled yarn was able to elongate to 22.36% structural strain from 5.27%. During the first heating cycle, the coiled yarn was only able to recover back to 11.91% structural strain. The remaining 10.45% of unrecoverable structural strain can be attributed largely to internal friction along with a combination of structural shakedown, and potential overstressing of certain filaments.

FIG. 14B is a graph of actuation contraction results from the test illustrated in FIG. 14A. Even with internal friction playing a role, the coiled yarn was able to achieve a maximum average actuation contraction of 10.65% at a constant force of 4.0N. However, the coiled yarn produced a maximum average specific work of 778.8 J/kg at a constant force of 6.0N. In actuating systems that integrate artificial muscles, the actuation contraction is critical for designers to achieve desired motions under application-level loads.

FIG. 14C is a graph of free displacement testing results to quantify the actuation contraction potential of an 800-10 OTC yarn. The OTC yarn achieved a maximum average actuation contraction of 10.65% at a constant force of 4.0N. Additionally, it is promising for application integration that the actuation contractions at the neighboring 2.0N and 6.0N holds were still significant (8.55% and 8.19%, respectively). The OTC yarn produced a maximum volumetric work density of 1551 kJ m⁻³ at a constant force of 6.0N. For comparison, the typical and maximum biological muscle work densities are 8 and 40 kJ m⁻³, respectively^([4]). The robustness of the OTC yarn to produce large actuation contractions and work densities over a range of forces is promising for applications in which a desired motion is required for an unspecified or varying applied load. In other material systems, maximum actuation contraction typically occurs at minimal applied stresses, but for SMAs, the actuation mechanism is driven by the phase change from M^(d) to A. The M^(d) phase is more present in the material under an applied load, which is why the maximum actuation contraction of the 800-10 NiTi OTC yarn is at an intermediate loading of 4N applied force. Large actuation occurs under higher, application-level forces offering a more dynamic performance than traditional artificial muscles, which produce maximum actuation displacements under minimal forces.

FIG. 14D is a graph of force blocking testing results that include scalable generated forces exhibited through manipulation of bundle configurations in a 400-10, 800-10, and 3200-5 OTC yarns. Demonstrating feasibility in force generating applications, the 800-10 OTC yarn was capable of producing a maximum of 8.12N during a transformation that occurred at 30% structural strain. The generated forces can be tuned and scaled for specific applications by manipulating the amount of active material. For example, the 400-10 OTC yarn, which has half the amount of active material of the 800-10 OTC yarn, exhibited a maximum force generation of 4.23N during a transformation at 25% structural strain. Meanwhile, a 3200-5 OTC yarn, which has the same amount of active material as an 800-10 configuration, demonstrated only 6.22N at 20% (compared with 7.57N in the 800-10 OTC yarn). This decreased force capacity is likely a result of amplified structural friction and surface interactions within the OTC yarn from the increase of filaments and decrease of filament diameter. Scalable forces are important for force-generating applications in which specific force ranges are desired to avoid system failure (over-stressing) and underperformance.

Shape Memory Effect Damping Performance of NiTi Over-Twisted Coiled Yarns

Superelastic SMA exhibits a large hysteresis through the forward and reverse A ⇄M^(d) transformations. The hysteresis, which is mechanical energy that is absorbed and dissipated as heat, can be leveraged in damping applications^([49-54]). The specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), is commonly used to compare damping capabilities across SMA structures, such as illustrated in FIG. 14E. Compared with monofilament SMA, which possesses a superelastic loading SDC of 23%, the NiTi OTC yarns exhibit an increased SDC of 61.0%, such as illustrated in FIG. 13B, indicating the NiTi OTC yarn may have significant damping potential. To explore the damping potential further, oscillatory tests were performed to 1) quantify the damping performance through SDC metrics and 2) investigate known SMA damping dependent variables such as oscillation amplitude strain, oscillation frequency, and the structural pre-strain within the over-twisted coiled yarn structure, such as illustrated in FIGS. 14E and 14F. In general, NiTi OTC yarns exhibited increased SMA damping performance (>20.0%, as illustrated in FIGS. 14E and 14F below) compared with commercial NiTi wire (typical SDCs in the 5%-10% range).

Oscillatory tests were performed on the coiled yarns on an RSA-G2 DMA. The coiled yarns were fixed within the environmental chamber, heated to austenite, and the austenite free length was found at a light tension of 10 grams. The coiled yarn was strained to a pre-strain and held. Dynamic strain loading was applied to the yarn at a preset oscillation frequency and oscillation amplitude strain for 100 cycles. For the oscillation frequency sweep, the oscillation amplitude strain was a constant 2.5%. Conversely, the constant frequency for the oscillation amplitude sweep was 1 Hz. For calculation of the SDC, the last cycle (n=100) was analyzed to determine the hysteresis area and area under the loading curve. Additional frequencies or amplitudes are applied at this pre-strain before strain loading to the next incremental pre-strain.

FIG. 14E is a graph of specific damping capacity at various oscillation amplitude strains. Damping behavior in the coiled yarn remained strongly tied to oscillation amplitude strain. Damping behavior in the OTC yarn remained strongly tied to oscillation amplitude strain (estimated to be 3.0% to 12.0% structural strain, as shown in FIG. 14B). Across all pre-strains, the SDC performance was greater at larger amplitudes, which gave the OTC yarn a larger opportunity to leverage the A→M^(d) transformation (22.5% average increase in SDC across amplitude range). In general, this supports the findings from FIG. 14B that a majority of A→M^(d) transformation occurs from ˜3.0% to ˜12.0% structural strain. For similar reasons, the structural pre-strain plays a role in performance because damping behavior is dependent on the A→M^(d) transformation. Across most oscillation amplitude strains, as illustrated in FIG. 14E, the SDC values of the 10.0% pre-strain exhibit increases from the 5.0% pre-strain and 15.0% pre-strain until 3.5% oscillation amplitude strain. The 10.0% pre-strain exhibits increased performance because it is at the end of the A→M^(d) transformation range where most of the material has undergone transformation that can be leveraged, compared with 5.0% (beginning of A→M^(d) transformation), and 15.0% (post A→M^(d) transformation). At 3.5% oscillation amplitude strain, the strain range for the 10.0% pre-strain is pushed beyond A→M^(d) transformation resulting in diminished SDC performance compared with the 5.0% pre-strain.

FIG. 14F is a graph of specific damping capacity at various oscillation frequencies. In support of these findings, in the frequency sweep results, the 10.0% pre-strained coil exhibits elevated SDC's compared with the 5.0% and 15.0% pre-strains. Additionally, it was found that in the OTC yarn structure, damping behavior was uninfluenced by oscillation frequency exhibited by the 4.84% average change in SDC from 1 to 10 Hz across all pre-strains. The lack of frequency-dependent behavior observed suggests the inclusion of NiTi within the OTC yarn structure eliminates known NiTi strain-rate dependent behavior^([33]). The NiTi OTC yarns exhibit multifunctional damping behavior, not seen in other OTC yarn work, including enhanced SMA damping performance, tunable performance through oscillation amplitude strain and pre-strain, and diminished frequency dependent behavior.

Coiled Yarn Textile Manufacturing

As standalone structures, OTC yarns offer large actuation contractions and generated forces for artificial muscle applications. Additionally, they are 1D structures that are integral for textile manufacturing. Woven and knitted textiles were manufactured to demonstrate the potential of OTC yarn textiles for actuating applications. For the woven garment, eight 230 mm long OTC yarns consisting of 800-10 microfilament bundles were manufactured. FIG. 15A is an image of manufacturing of an 800-10 OTC yarn woven textile in a loom. The eight OTC yarns were fixed in parallel 8.5 mm apart in the warp (y) direction under a light tension (50 g). FIG. 15B is a cross-sectional diagram of a woven structure in which OTC yarns are interwoven by Kevlar thread. A Kevlar yarn was woven through the OTC yarns in the perpendicular weft (x) direction, providing structural integrity to the woven textile. The Kevlar was chosen for the weft material for its small thermal expansion coefficient, which isolates the performance of the NiTi OTC yarns during thermal actuation testing. FIG. 15C is an image (left) of an OTC yarn woven textile amidst manufacturing, and a close-up image (right) of interaction between OTC yarn and Kevlar within the woven. The final woven textile measured 57×160 mm, but alterations to the number of OTC yarns, warp spacing, and loom length could tailor woven dimensions.

FIG. 15D is an image of manufacturing of a 400-10 OTC yarn garter knit textile on a weft knitting machine. The knitted textile was manufactured on a manually driven Taitexma TH-160 with a fixed distance of 6 mm between the knitting needles using 400-10 OTC yarns configurations. The OTC yarn samples were joined in length with a sacrificial Kevlar thread to protect the OTC yarns from large de-coiling forces applied from the knitting machine. Together, the OTC yarn and Kevlar thread were knitted into a garter pattern knitted textile with alternating rows of purl and knit loops. FIG. 15E is a diagram of a unit loop of an OTC yarn garter knit to demonstrate the path of a 1D over-twisted coiled yarn within a larger structure. The garter textile pattern was chosen to isolate linear actuation performance in traditional monofilament NiTi textile. FIG. 15F is an image (left) of a final garter patterned knitted textile composing of 400-10 OTC yarn, and a close-up image (right) of interaction between the unit loops. The Kevlar thread was cut away post knit manufacturing, leaving the OTC yarn garter textile and final dimensions of 37 mm×65 mm.

Coiled Yarn Textile Performance

The OTC yarn textile actuation performance was encouraging for various artificial muscle applications, especially medical compression garments. Ideal compression garments must undergo large structural strains to stretch over areas of the body, exhibit a low force response in the inactivated state for ease of use, and provide large, scalable generated forces during activation to apply medical-grade pressures to the user. FIG. 16A is a graph of force block results of the 800-10 OTC yarn woven textile, while FIG. 16B is a graph of force block results of the 400-10 OTC yarn knitted textile. Both textiles could undergo large structural deformations, up to 55.0% in the woven textile until failure and 180.0% in the knitted textile without failure. This is a drastic improvement over single OTC yarns, which typically fail in the 20%-40% structural strain range, such as illustrated in FIG. 13A, and directly applies to the ability of the OTC yarn textiles to be stretched over a large area of the body during donning. In single OTC yarns, the primary mechanism of failure occurs when the axial tension pulls the NiTi microfilaments out of the OTC yarn architecture back into a twisted yarn. The yarn section becomes the weakest point and fails with additional loading.

There is a combination of reasons for the extended strain range of the woven textile compared with a single OTC yarn. First, the OTC yarns are in parallel, sharing the total applied load across each OTC yarn. It is assumed that due to manufacturing inconsistencies, the load is not split evenly across each OTC yarn. It is theorized that the load is applied across a subset of the OTC yarns. When those heavily loaded OTC yarns begin to weaken, either from the A→M^(d) transformation or partial de-coiling, the load is transferred to adjacent OTC yarns that were previously unloaded. This form of load sharing allows the entire structure to be strained to large structural strains, beyond that of a single OTC yarn. It also explains why the austenitic forces of the woven textile, as shown in FIG. 16A, are only 4-5 times stronger than the single OTC yarn, as shown in FIG. 16B, despite the seven additional coils present in the woven textile. Additionally, it is theorized that the Kevlar weft yarn applies compression to the OTC yarns, providing a resistant force against de-coiling. Meanwhile, in the knitted textile, structural displacement is converted to the reorientation of the constitutive loops as well as bending, torsional, and axial stressing of the OTC yarns. This results in the 350% percent increase in allowable structural strains exhibited in the knitted textile compared to the standalone OTC yarn.

Additionally, during donning of a compression garment, it is desirable for the compression garment to provide the least amount of resistance to make the experience easier on the user. For an SMA-based compression garment, donning would occur in the less stiff martensitic phase. Encouragingly, both textiles demonstrate a low force response in the martensitic state, such as illustrated in FIGS. 16A and 16B. At low structural strains (0-10.0%), the woven martensite response maintains forces below 1.5N, compared with 31.4N in austenite, a 20.9 times change in force between the two states, compared to just a 2.27 times change in force for the single OTC yarn at 10.0% structural strain. Additionally, the knitted textile martensitic response maintains low forces throughout reorientation, staying below 1.3N up to 90.0% structural strain.

FIG. 16C is a graph of the generated force in the woven textile, knitted textile, and constitutive 1D 800-10 and 400-10 OTC yarns. Upon heating through the austenite transformation, the textiles generated large forces, up to 53.3N at 40.0% structural strain in the woven textile and 59N at 180.0% structural strain in the knitted textile. This aspect directly relates to generating pressures needed for a medical benefit to the user. A specific example of a high-pressure on-body compression application is a mechanical counterpressure suit, which requires 29.6 kPa of pressure for astronauts to survive the vacuum of space. On-body pressure (P) is calculated using Laplace's Law, which is derived from pressure vessel theory, and can be described as the ratio of generated force per unit width (F_(w)) and the radius of the cylinder (body part). At the current maximum generated forces, the woven and knitted textile could provide sufficient counterpressure (>29.6 kPa) for body parts with a radius of ˜3.0 cm, such as the forearm. The knitted and woven textiles were not optimized for maximizing generated forces, the generated force per unit width (F_(w)), or on body pressure. Pressures could significantly improve through a variety of tunable parameters such as bundle configuration (filament number and diameter), the number of woven warp muscles, loom warp spacing, and knit bed needle spacing. A counterpressure suit capable of life-sustaining pressures in the vacuum of space is achievable from tailored NiTi OTC yarn muscle technology.

The generated force profiles of the two textiles are qualitatively different. The woven textile exhibits immediate large, generated forces at relatively low structural strains, whereas the generated forces of the textile do not significantly increase until 70.0% structural strain. During early loading of the knitted textile, reorientation (sliding) of the OTC yarns mitigates the strains and stresses applied directly to the OTC yarn. Without any strains, the OTC yarns exhibit limited strain recovery during the austenitic transformation resulting in low generated forces. The two profiles could be used to tailor behavior within segments of a medical compression garment to apply pressure to the user under a small donning pre-strain, or a large pre-strain from donning over a large part of the body. The textiles offer an exciting combination of tailorable, scalable, and ideal force generation solutions for actuating applications driven by moderate thermal stimulus.

Example Textile—Actuating Compression Garment

Multifunctional SMA cloth-like textiles may be used for both active and passive applications. Unlike previous SMA monofilament textiles, the microfilament yarn-based systems exhibit strong wearability and tunable performance. The garter textile designed for active compression wearables exhibits a low-force martensitic response for donning, a large structural strain ranges for stretching over large parts of the body, and generated forces for beneficial therapeutic compression. The generated forces can be scaled for specific applications through an increase in the amount of active material without sacrificing manufacturing abilities. Additionally, the altered deformation modes and fundamental understanding of SMA microfilament yarn mechanics promotes predictable behavior in the textile architecture.

To demonstrate the damping multifunctionality of SMA, a 3D spacer fabric was manufactured for integration within prosthetic attachment systems. The 3D spacer fabric absorbs and dissipates mechanical energy through a combination of inter-yarn friction and SIM transformation mechanisms. The damping ability of the fabric is dependent on the pre-strain and oscillation amplitude, which control the amount of SIM transformation. The performance of the spacer fabric can be tuned for specific user activities through changes to the spacer thickness, spacer yarn pattern, and spacer yarn configuration without sacrificing manufacturing. The work presented showcases the improved wearability and tunability of SMA multifunctional textiles through the inclusion of SMA microfilament yarns and establishes the foundation for optimization of SMA textiles for application-specific performance.

Functionalized SMA Microfilament Fabrics

Multifunctional SMA cloth-like textiles may be used for both active and passive applications. Unlike previous SMA monofilament textiles, microfilament yarn-based systems exhibit strong wearability and tunable performance. As will be illustrated below, such SMA multifunctional textiles may have improved wearability and tunability through the inclusion of SMA microfilament yarns and may be improved or optimized for application-specific performance.

As one example textile, a garter textile designed for active compression wearables may exhibit a low-force martensitic response for donning, a large structural strain ranges for stretching over large parts of the body, and generated forces for beneficial therapeutic compression. The generated forces can be scaled for specific applications through an increase in the amount of active material without sacrificing manufacturing abilities. Additionally, the altered deformation modes and fundamental understanding of SMA microfilament yarn mechanics promotes predictable behavior in the textile architecture.

As another example textile, a 3D spacer fabric for integration within prosthetic attachment systems may exhibit damping multifunctionality. The 3D spacer fabric may absorb and dissipate mechanical energy through a combination of inter-yam friction and SIM transformation mechanisms. The damping ability of the fabric may be dependent on the pre-strain and oscillation amplitude, which control the amount of SIM transformation. The performance of the spacer fabric can be tuned for specific user activities through changes to the spacer thickness, spacer yarn pattern, and spacer yarn configuration without sacrificing manufacturing.

Example Textile—Actuating Compression Garment

One example textile described herein includes a highly tunable and scalable SMA microfilament yarn integrated garment that exhibits a traditional clothing-like aesthetic, a low force inactivated state, and generated pressures through the SME. For example, on-body compression garments may be used for a wide range of applications, from everyday wearables to performance-enhancing athletic clothing to therapeutic garments. Compression is used in conjunction with wearables such as fitness trackers to ensure a good connection between sensors and users. Athletic garments use targeted compression to promote recovery, prevent injuries, and improve athletic performance. In medical devices, compression may be used to control excessive swelling in lymphedema patients and promote blood flow to prevent deep vein thrombosis (DVT) and orthostatic hypotension (OH).

Current on-body compression technologies typically use passive systems with elastic materials that can undergo large structural strains to stretch over a portion of the body and elastically recover applying pressure to the user. However, elastic materials and structures, which follow Hooke's law, provide continuously increasing pressure with added structural strain making it difficult to achieve specific user pressures across different anthropometric makeups. Additionally, these garments are significantly stretched during donning, requiring large forces from the user, of which, pose a problem for elderly and disabled users. To overcome the forces needed for donning, some current compression technologies utilize an activated pneumatic system. These pneumatic-based devices are easy to don and can provide significant pressures when activated but are large, bulky, loud, and prevent user mobility.

Through the shape memory effect (SME), SMA integrated textiles can undergo large, activated contractions, providing significant on-body pressures to a user. Additionally, the inactivated, less stiff martensitic state provides a low force response ideal for the user experience during donning. However, current SMA compression garments composed of large monofilaments limit their wearability, manufacturability, and performance tunability. SMA microfilament yarns have been demonstrated to maintain strong SME behavior in a highly tunable, flexible architecture ideal for dense knitting manufacturing processes.

Example Textile—Energy Absorbing Prosthetic Sock Liner

Another example textile described herein includes a 3D SMA integrated spacer textile that combines the high stiffness, large recoverability, and hysteretic damping of superelastic SMA in a lightweight textile architecture designed for mechanical energy absorption that may be optimized to mitigate interfacial pressure points afflicting prosthetic users. For example, there remain many difficulties creating properly fitting prosthetics, plaguing lower leg amputees and inhibiting a successful rehabilitation. Most of the pain and discomfort stem from interfacial pressure points because of the varying stresses from daily activities and bodily volume fluctuations. Current technologies emphasize the customized fit of the prosthetic socket system to mitigate pressure points, but these systems require constant fit adjustments and have trouble accounting for bodily volume fluctuations.

As a multifunctional material system, shape memory alloys have unique damping and energy-absorbing properties that make them a natural solution in prosthetic attachments. For example, the constant stress plateau during austenite to detwinned martensitic transformation can be leveraged to provide consistent pressures to a user's unique body contour and over large range of displacement fluctuations. Additionally, superelastic SMA behavior is a hysteretic process where mechanical energy is dissipated as heat—a fundamental characteristic for damping. Leveraging the transformation of energy as a damper in a prosthetic attachment could lessen the pressures felt by the user during strenuous activities. However, prosthetic attachments require lightweight solutions and as a metal alloy, solid SMA structures are typically heavy and dense. For integration in prosthetic attachments, a lightweight yet strong SMA architecture is needed.

Spacer textile architectures provide lightweight solutions to integrate materials in a 3D energy-absorbing structure. Spacer textiles consist of 2D knitted surfaces that are joined by a perpendicular spacer yarn. Spacer textiles geometrically leverage the reorientation and complex loading (bending, shearing, torsion) of the spacer yarns in compression to absorb large amounts of mechanical energy. Spacer yarns typically consist of highly elastic polymers that can undergo deformations and linearly recover —springing the textile back into its original shape. However, these materials are ideally used in low-force applications as higher forces will cause permanent densification and deformations of the textile. Additionally, the thickness of the spacer fabric is limited by strains put on the spacer yarn to avoid permanent deformations during compression.

Example Textile Manufacturing

The SMA microfilament yarns were manufactured at a local spinning mill on industrial ring spinners capable of controlling the delivery speed, spindle angular velocity, and inserted twist. Two yarn variants were manufactured: a 400 filament bundle with 5 μm diameters with a twist of 10.53 TPCM used in the spacer fabric, and a 400 filament bundle with 10 μm diameters with a twist of 5.27 TPCM used in the compression garment. For both yarns, heat treatment was necessary for stabilizing transformations, shifting transformation temperatures, and twist setting the austenitic memorized shape. The yarns were thermally processed under the same conditions (11 minutes at 550° C.) after twist insertion (Thermal Processing). The processed yarns were manufactured into either a 2D garter textile for actuating applications or a 3D spacer textile for energy absorbing applications.

FIG. 17A is diagram of a garter knitted textile composed of SMA microfilament yarns for applications in active compression garments applications. The garter textile was knitted on a Stoll CMS 303 machine with a spacing of 12 needles per inch. Garter textiles are composed of alternating rows of knit and purl loops. The alternating pattern gives the textile a front/back symmetry not achievable in simpler stockinette textiles. The symmetry is important for isolating thermal actuation contractions that occur planar as opposed to the asymmetric stockinette, which contracts in plane and furls out-of-plane.

FIG. 17B is a diagram of a 3D spacer textile with Kevlar knitted surfaces and SMA microfilament spacer yarn for potential use in prosthetic socket liner applications. The spacer textile was also knitted on a Stoll CMS 303 with a spacing of 12 needles per inch. The spacer textile is composed of two stockinette knitted surfaces made of Kevlar that are joined by the SMA microfilament yarn laid vertically across. Kevlar possesses a low thermal contraction coefficient, allowing any changes in structure to be isolated to the SMA spacer yarns.

Example Textile Testing and Characterization

Experimental characterization of the actuating garter textiles assessed the compression potential through force block testing. Force block testing was performed on an Instron 3365 within an environmental chamber. Force block testing is designed to mimic the user experience of an active compression garment. Donning of the garment was assumed to be performed in the flexible martensitic state. Stretching in the lower martensitic stiffness corresponds to a low force response during donning, allowing a user to deform the garment over a section of the body easily. In force block testing, this is captured by an initial strain-controlled loading from unstressed twinned martensite. After donning, the garment will be thermally activated (passively from body heat or through a controlled input) to the stiffer austenitic state. Upon transformation to the stiffer austenitic state, the garment will undergo an increase in force from recovery stress generation (RSG). The difference in force between the donned martensitic state and the activated austenitic state represents the generated forces felt by the user. In force block testing, this is characterized by iso-strain thermal cycles to measure the generated forces. The garment is incrementally strained and thermally cycled to characterize the force-generating abilities for different stretches and areas of the body (structural strains). On-body pressure (P) is calculated using Laplace's Law, which is derived from pressure vessel theory and can be described as the ratio of generated force per unit width (F_(w)) and the radius of the cylinder (body part).

Energy absorption and damping potential of the 3D spacer textile are characterized through a combination of isothermal quasistatic isothermal compression and dynamic oscillatory tests. The quasistatic isothermal compression tests were performed on a DMA RSA-G2 within an environmental chamber at temperatures either greater than A_(f) or below M_(f). The typical compression behavior of a spacer fabric is split up into three distinct stages. The first is a linearly sloped region, which corresponds to compression and light buckling of the spacer yarns. The second stage corresponds to a nearly constant stress plateau, which is a result of additional buckling, rotating, and shearing of the spacer yarn. This is followed by a steep increase in stress from the densification of the fabric, which restricts any further movement of the spacer yarn. Utilizing the constant stress plateau is critical for prosthetic attachments applications where consistent reaction forces are ideally applied to the body over a wide range of displacements. Isothermal compression tests are performed to characterize the compression behavior and identify the staged behavior of SMA microfilament spacer textiles.

Oscillatory testing captured the dynamic response to varying loads. Oscillatory tests were performed on a DMA RSA-G2 by compressing the textile to a pre-strain and oscillating displacements at a controlled oscillation amplitude and frequency. The corresponding force-strain data was analyzed for mapping the force response theoretically felt by the user and measuring the mechanical energy that is dissipated as heat from internal friction and martensitic transformation. A common way to quantify SMA's damping effectiveness across diverse architectures is through the specific damping coefficient (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve).

Actuating Performance of Cloth-Like SMA Garter Textile

Without optimization, the cloth-like garter textile exhibited behaviors ideal for active compression wearables and expanded tunability for optimized performance compared to previous monofilament-based systems. In monofilament-based SMA knitted textiles, the force response to structural strain is dependent on the high bending stiffness and inter-monofilament frictional behavior. Upon loading, an initial reorientation of the monofilaments through interfacial sliding occurs before interlocking due to the build-up of stick frictional forces. While sliding enables large structural straining of the knitted textiles, force generation is limited in this stage. In the absence of sliding, the recovery of stresses from bending deformations from the interlocked loop geometries drives an increased force generation during thermal heating. With additional loading, the recovery of stresses is not enough to overcome the strained loop geometry, resulting in a decrease in force generation.

FIG. 18A is a graph of force block results of the cloth-like SMA knitted garter textile. SMA knitted textiles composed of microfilament yarns undergo similar staged behavior. Initial loading exhibited reorientation of the microfilament yarns through interfacial sliding. In this reorientation stage (0-90% structural strain), generated forces are small as direct stressing of the microfilaments remains limited. The broad reorientation strain range coupled with the low martensitic force response are ideal characteristics for an easier user experience during donning of a compression garment. Additionally, the tunability of this reorientation strain range is critical for designing compression garments that stretch precisely over specific portions of the body. In monofilament textiles, the reorientation stage dependent on the density (knit index) of the textile. However, the densities of monofilament SMA textile systems are limited by the high bending stiffness of the wire, which limit specific tunability in compression garment applications. Conversely, the compliant nature of the SMA microfilament yarns permits limitless tuning of the textile density, and thus, the textile behavioral response for donning on specific portions of the body.

FIG. 18B is a graph of generated forces from the thermal heating ramps of the cloth-like SMA knitted garter textile. At ˜90% structural strain, the generated forces begin to increase rapidly, suggesting that stick frictional forces between the yarns are replacing sliding. The interlocked state correlates to more direct stressing of the SMA microfilaments for recovery in RSG. The generated forces reach a maximum of 1.95N (135% structural strain, but with no peak force generation reached, further testing at higher structural strains is needed to fully understand the force generation performance of microfilament yarns within active contractile textiles. Normalizing by the dimensions of the textile, this maximum generated force correlates to an on-body pressure of 6.08 kPa, which is enough to provide therapeutic benefits to users with disorders such as orthostatic hypotension (OH) (3.3-5.3 kPa) but insufficient for DVT prevention (17.3-26.6 kPa). For large desired pressures applications such as DVT prevention, scaling in monofilament SMA systems is mainly done by increasing the amount of active material through larger monofilament diameters. However, larger filament diameters correlate to significant increases in bending stiffness that prevent their integration in knitted textile architectures. For SMA microfilament yarns, scaling the amount of active material does not significantly impact the bending stiffness permitting the creation of dense, cloth-like textiles with differing amounts of active material.

Additionally, the large reduction in bending stiffness in the microfilament yarns drastically alters the dominating underlying mechanisms used to generate forces during RSG. Instead of leveraging mainly loop geometry bending deformations in the interlocked state (stick frictional forces), the SMA microfilament yarns leverage a combination of microfilament helical deformations within the yarn (tension, compression, bending, torsion) from direct yarn axial straining during the interlocked stage. The highly compliant yarn structure enables tight loop geometries in the interlocked state, which results in the microfilament helical deformations from axial loading dominating RSG behavior. This loading of microfilament yarns is well understood through previous experimental and modeling works, permitting tunable predictive behavior of SMA microfilament yarns and textiles. The cloth-like, easily tunable, and scalable SMA yarn knitted structure exhibits significant improvements over monofilament SMA textiles, advancing the functionalization of this technology for on-body compression.

FIG. 18C is a graph of isothermal quasistatic compression results of the spacer textile at a temperature above AF and below MF. The NiTi yarn spacer fabric did not exhibit typical spacer fabric compression behavior; however, it exhibited three distinct stages of compression behavior. The first stage (stage I, 0% to ˜16% compression strain) corresponds to a combination of initial fabric engagement and austenitic loading (curve I) of the SMA spacer material. At ˜16% compression strain (stage II), there is a slight change in stiffness, which is hypothesized to be a result of SIM transformation and filament reorientation through shearing, rotation, and buckling. While the change is minor, the presence of SIM transformation is critical for maximizing energy dissipation and strain recoverability. The combination of SIM transformation and filament reorientation continues until ˜29% compression strain where densification of the spacer fabric dominates, exhibited by a sharp increase in force (stage III). If any further SIM transformation is occurring in the NiTi yarns in this stage, the contribution in the textile force response is being dominated by the densified state of the spacer fabric.

Because martensitic reorientation is a one-way phenomenon, the martensitic experimental curve (curve II) is treated as a passive material system, offering significant insight into the impact of the superelastic effect. For example, the mechanical energy absorbed and dissipated is quantified by the area (hysteresis) between the loading and unloading curves. In austenite, the spacer fabric exhibited a large hysteresis between the loading and unloading curve, resulting in 740.2

$\frac{kJ}{m^{3}}$

of dissipated energy per unit volume. The energy dissipated in austenite is a combination of reversible SIM transformation of the SMA material and friction within the yarn and spacer fabric structures. To isolate the impact from SIM transformation, a comparison with the compressional loading in martensite was performed. The spacer fabric martensitic curve exhibited a hysteresis corresponding to 494.4

$\frac{kJ}{m^{3}}$

of dissipated energy per unit volume. This can predominately be attributed to the inter-filament and inter-yarn friction within the fabric system.

The inner loop hysteresis can also be analyzed through damping efficiency metrics. The specific damping capacity (SDC), defined as the ratio of dissipated energy (hysteresis area) to stored energy (total area under loading curve), is commonly used to compare damping capabilities across SMA structures. Compared with monofilament SMA, which possesses a superelastic loading SDC of 23%, the SMA spacer fabric exhibits an increased SDC of 59.5%, which indicates a high damping potential by the SMA within this architecture.

Additionally, the impact of superelastic NiTi can be observed in the strain recoverability of the spacer fabric in austenite compared with martensite. In austenite, the spacer fabric recovered 89.3% of the applied strain, compared with 75.7% in the martensitic curve shown in FIG. 18C. While strain recoverability is contributed partly by textile structural effects exhibited in the martensitic strain recovery, the increased strain recovery in austenite agrees with the previous findings that forward and reverse SIM transformation is occurring and contributing within the spacer textile structure.

FIG. 18D is a graph of oscillatory results of the spacer textile at varying pre-strains (10, 20, 30, 40, 50, corresponding to curves I, II, III, IV, V) and oscillation amplitude strains. Oscillatory testing was performed to mimic walking loads on a prosthetic socket liner and provide an understanding of the performance of the SMA yarn spacer fabric in the context of this application. Oscillatory displacement tests were performed at a constant oscillating frequency of 1 Hz through a sweep of oscillating amplitude strains at different compressional pre-strains. The frequency could be thought of as the walking gait pace. The differing amplitude strains account for variations in loads from different terrain topologies, surface hardness, and residual limb volume fluctuations. Lastly, the pre-strains account for different user weights in an idled position.

The SMA spacer fabric exhibited known SMA damping behavior highly dependent on the pre-strain and amplitude strain. In general, the NiTi Spacer fabric exhibited increased SMA damping performance (>20.0%) compared with commercial NiTi wire (typical SDCs in the 5%-10% range). The best damping performances (SDC-25%) occurred under a light pre-strain (10-20%) and high oscillation amplitude strain (7.5-10%). Under these conditions, the oscillatory loading range stays consistently in Stage I and II behavior, where it was initially identified that SIM transformation might be occurring. These results suggest optimization of the spacer fabric comes from close coordination between initial fabric behavior (filament engagements and reorientation) and loading of the spacer yarn (SIM transformation).

The goal of integrating SMA microfilament yarns within a spacer fabric was to leverage both material and structural behaviors to improve spacer fabric performance. While the spacer fabric presented in this work does not exhibit an ideal constant-force plateau, it successfully leverages SIM transformation within a 3D textile to improve strain recoverability and energy dissipation over passive material systems (martensite). The spacer fabric exhibits damping efficiencies dependent on pre-strain and strain amplitude that can be tailored for specific applications such as prosthetic attachments. Future iterations may optimize the relationship between filament reorientation and SIM transformation through alterations to the yarn configuration, spacer fabric thickness, and spacer yarn pattern.

Energy Absorbing Textiles

3D spacer fabrics with superelastic materials can provide constant force profiles, enhanced damping frequency ranges, and large energy dissipations for prosthetic attachments, helmet technology, and impact resistance fortifications. Usually, researchers manufacture multifunctional textiles with monofilament and yarns but recently, over-twisted coiled structures have demonstrated improved actuation contractions, force generations, and strain recovery. For energy absorption, the NiTi over-twisted coiled yarns were structured within a 3D spacer textile structure, and experimentally investigated quasi-statically for strain recovery and energy absorption through hysteresis, as well as dynamically for mapping damping performance dependent behavior.

Textiles describes herein may be manufactured from NiTi microfilament over-twisted coiled yarns for medical device applications such as medical compression garments, as well as energy absorbers for defense, impact, and damping applications. Over-twisted coiled yarn structures are created on a custom-built manufacturing system by inserting twist into a bundle of NiTi microfilaments until the torsional imbalance imparted on the yarn causes coils in the direction of the yarn axis to form. The coiled yarns are thermally processed to shift transformation temperatures and shape set the final coiled structure.

To demonstrate the superelastic and energy absorbing potential, a textile in the form of a 3D spacer fabric was manufactured. A 3D spacer fabric was manufactured with energy absorbing applications in mind such as impact absorption, and vibrational damping. 3D spacer fabrics consist of two outer knitted layers connected and distanced by a spacer yarn, which gives the fabric it's 3D form. The 3D spacer fabric leverages both traditional structural effects as well as the unique energy absorbing behaviors of superelastic NiTi. Isothermal compression tests were performed to quantify energy absorption and understand coiled yarn spacer fabric kinematics, while dynamic oscillatory tests were performed to map relationships between pre-strain and frequency to loss modulus and tan delta. The NiTi coiled yarn spacer fabric exhibited potential as a high force energy absorber and future optimizations are discussed to improve performance.

Spacer Fabric Manufacturing

Coiled yarns were manufactured as described in FIGS. 9A-9C above. FIG. 19A is an image (left) of a spacer fabric manufacturing process, and a diagram (right) of the spacer yarn pattern integrated across two Kevlar stockinette surfaces. The weft knit energy absorbing spacer fabric was manufactured on a manually driven weft knitting machine Taitexma TH-260 with a fixed distance of 9 mm between the knitting needles and a fixed distance of 12 mm between knitting beds.

FIG. 19B are images of (left) a side view and (right) a top view of a finished spacer fabric. The top and bottom fabric surfaces were knit with Kevlar yarn in a stockinette (jersey) pattern on two needle beds. The spacer yarn consisted of the over-twisted NiTi coiled yarns and was wrapped in a crossing pattern in the x-z plane joining the two stockinette surfaces, as shown in FIG. 19A. The thickness of the spacer fabric was dictated mainly by the spacing between the two needle beds. The final dimensions of the spacer fabric measured 40×30×10.5 mm in a stress-free austenitic state.

Spacer Fabric Testing and Characterization

The energy absorption potential of the spacer fabric was investigated through slow, displacement controlled isothermal compression tests as well as dynamic testing performed on a TA Instruments RSA-G2 dynamic mechanical analyzer (DMA). Displacement controlled compression tests were performed at 80° C., above A_(f), at a global displacement rate of δL⁻¹=±4×10⁻⁴ s⁻¹ with 25 mm diameter compression plates corresponding to a testing area, A_(comp), of 4.908×10⁻⁴ m². Displacements and forces are converted to structural strain and pressure to quantify common spacer fabric and engineering metrics. In spacer fabric technologies, it's common to analyze the loading curve for the shape and amount of energy absorbed per unit volume (W), defined as the area under the pressure-strain loading curve. However, analysis of the unloading behavior is needed to understand the NiTi material impact within the spacer fabric structure. The dissipated energy per unit volume is defined as the area between the loading and unloading curve and offers insight into the forward and reverse martensitic transformations occurring in the NiTi coiled yarns during loading and unloading.

Lastly, dynamic mechanical testing was performed to isolate the vibration damping potential of NiTi coiled yarns. The damping performance of NiTi structures is known to be dependent on the prestrain, vibration amplitude, and vibration frequency. Dynamic testing consisted of loading the NiTi coiled yarn sample to a fixed prestrain, before cycling through an oscillation strain with a corresponding amplitude and frequency. Oscillatory testing is used to quantify tan delta (tan δ), where delta (δ) is the phase lag between input strain and resulting stress. The tan delta, also known as the loss tangent, is commonly used as a key metric to analyzing damping performance. Tan delta is commonly used as an efficiency metric but does not offer much insight into the amount of energy that is being absorbed and dissipated; instead, the loss modulus represents the amount of energy absorbed and dissipated as heat.

Spacer Fabric Compression Behavior

Typical compression behavior of a spacer fabric is split up in four distinct stages. The first is a lower sloped linear region where the loose outer layers being engaged. This is followed by a higher linearly sloped region which corresponds to compression and light buckling of the spacer yarns. The third stage corresponds to a nearly constant stress plateau which is a result of additional buckling, rotating, and shearing of the spacer yarn. This is followed by a steep increase in stress from the densification of the fabric which restricts any further movement of the spacer yarn.

The NiTi coiled yarn spacer fabric did not exhibit typical spacer fabric compression behavior; however, it did still exhibit three distinct stages of compression behavior. FIG. 20A is a graph of a loading curve of the NiTi coiled yard spacer fabric in austenite phase, including an initial engagement stage (I), a structural reorientation stage (II), and a densification stage (III). The first stage, I, (0% to 18.0% Structural Strain) corresponds to a light interaction of the fabric being engaged together demonstrated by a modulus estimation of 27 kPa. This is followed by a slightly stiffer linear region, II, (18.0% to 37.0%) with an estimated slope of 136 kPa, which is a combination of structural buckling, shearing, rotation of the coiled yarn as well as any stress induced martensitic transformation that may be occurring. Like typical spacer fabric behavior, the last stage, III, is the densification of the spacer fabric where movement of the spacer yarns are prevented by interaction and a sharp increase in stress is exhibited. If any further detwinning occurred in the NiTi coiled yarns in this stage, it is being dominated by the densified state of the spacer fabric.

Analysis of the unloading behavior and comparison with the martensitic loading/unloading offers insight into the NiTi superelastic impact within the spacer fabric. FIG. 20B is a graph of loading and unloading curves of NiTi coiled yarn spacer fabric in austenite and martensite phases. In austenite, the spacer fabric exhibited a large hysteresis between the loading and unloading curve, resulting in 6.67 kJ m⁻³ of dissipated energy per unit volume. The energy dissipation is a combination of forward transformation within the NiTi material as well as friction within the coiled yarn and spacer fabric structures. To isolate the impact from forward transformation, compression loading in martensite was performed. During loading in martensite, NiTi undergoes an irreversible transformation from temperature induced martensite to stress induced martensite, upon unloading limited strain recoverability is observed. However, the spacer fabric martensitic curve still exhibited hysteresis corresponding to 3.99 kJ m⁻³ of dissipated energy per unit volume. This signifies that structural friction shares a significant role dissipating energy along with traditional NiTi superelastic behavior. Additionally, the impact of superelastic NiTi can be observed in the strain recoverability of the spacer fabric in austenite compared with martensite. In austenite curve (II), the spacer fabric recovered 47.8% of the applied strain, compared with 40.0% in the martensitic curve (I), as seen in FIG. 20B. While strain recoverability is dominated by structural effects (coiled yarn and fabric), superelastic behavior still plays a minor role.

One potential goal of integrating NiTi coiled yarns within a spacer fabric is to leverage both material and structural stress plateaus to improve spacer fabric performance. While the spacer fabric of the present example is dominated more by structural effects, other examples may leverage more of the active material behavior by increasing the spacer fabric thickness, and altering the spacer yarn pattern to induce more buckling, shearing, and rotation of the spacer yarn. Leveraging both a material and structural stress plateau could improve spacer fabric energy absorbing performance.

Spacer Fabric Dynamic Behavior

While quasi-static isothermal compression tests can give researchers a glimpse into the energy absorbing potential of a material or structure, dynamic oscillatory tests are more commonly used to characterize the damping performance. Six oscillatory tests with increasing oscillation frequencies and constant amplitudes are performed at five structural pre-strains to map the damping capability of the NiTi coiled yarn spacer fabric. It was found that the damping abilities were strongly correlated to the structural pre-strain applied to the spacer fabric, as illustrated in FIGS. 20C and 20D below.

FIG. 20C is a graph of oscillation test results on the NiTi coiled yarn spacer fabric illustrating the relationship between oscillation frequency (1, 2.5, 5, 10 20, 40, corresponding to curves I, II, III, IV, V, VI) and structural prestrain on the tan delta. The tan delta, which offers insight into the ratio of elastic to viscous behavior in the sample, saw a slight increase at small pre-strains (0-20% structural prestrain) before tailing off at larger pre-strains. In the superelastic SMA material system, viscous behavior can be attributed to the forward and reverse martensitic transformation occurring. This means that when tan delta is a maximum, the sample may be undergoing its largest amount of stressed induced martensitic transformation, and austenitic recovery from the oscillating strain. Understanding how to maximum tan delta and the viscous response of the NiTi coiled yarn spacer fabric is essential to finding a coiled yarn and spacer structure that utilizes both structural and material effects to absorb and dampen mechanical energy. Additionally, the tan delta response of the fabric had a slight dependence on the oscillation frequency. Higher frequencies (10-40 Hz) corresponded to higher maximum tan deltas at smaller pre strains (10.0%), as illustrated in FIG. 20C. On the other hand, lower frequencies were able to maintain tan deltas near their maximums before tailing off at pre-strains greater than 20%. The lower frequencies allowed more time for the sample to undergo forward and reverse martensitic transformation, resulting in stable tan deltas over a larger pre-strain range.

To investigate the actual amount of energy dissipated by the SMA response, the loss modulus was calculated for each oscillatory test. FIG. 20D is a graph of oscillation test results on the NiTi coiled yarn spacer fabric illustrating the relationship between oscillation frequency (1, 2.5, 5, 10 20, 40, corresponding to curves I, II, III, IV, V, VI) and structural prestrain on the loss modulation. The loss modulus offers insight into the amount of energy being dissipated as heat. It was found that while the frequency had little to no impact on the loss modulus, the pre-strain had a large influence. Almost exactly mimicking the quasi-static compression test, the loss modulus is relatively small and linear at lower pre-strains before undergoing a steep increase during the densification of the spacer fabric. This makes sense as the loss modulus is scaled by the pressure exerted on the spacer fabric. However, even though the amount of energy being dissipated as heat is largest at 40% pre-strain, we know from the tan delta results that the ratio between the viscous and elastic behavior is at its lowest. This means that an even greater amount of energy is being stored linear elastically.

The following are some examples described herein.

Example 1: A shape memory coil includes a coiled shape memory yarn having a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.

Example 2: The shape memory coil of example 1, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are different.

Example 3: The shape memory coil of example 1 or 2, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same.

Example 4: The shape memory coil of any of examples 1 to 3, wherein the plurality of microfilaments includes between about 10 and about 1000 microfilaments.

Example 5: The shape memory coil of any of examples 1 to 4, wherein the shape memory alloy comprises at least one of a nickel-titanium alloy or a copper-zinc-aluminum alloy.

Example 6: The shape memory coil of any of examples 1 to 5, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.

Example 7: A method for manufacturing a shape memory coil includes coiling a shape memory yarn to form a coiled shape memory yarn that has a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.

Example 8: The method of example 7, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.

Example 9: The method of example 7 or 8, further comprising twisting the plurality of microfilaments to define the twist direction.

Example 10: The method of any of examples 7 to 9, wherein the shape memory yarn is coiled around a mandrel to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are opposite.

Example 11: The method of example 10, further comprising heat treating the yarn to torque balance the yarn.

Example 12: The method of any of examples 7 to 11, wherein the yarn is coiled to create a torsional imbalance to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same.

Example 13: The method of example 12, further comprising plying the shape memory yarn with one or more shape memory yarns to torque balance the shape memory yarn.

Example 14: A textile includes a plurality of interlocked tows, wherein at least a portion of the plurality of interlocked tows comprises a plurality of shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy.

Example 15: The textile of example 14, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.

Example 16: The textile of example 14 or 15, wherein the plurality of shape memory yarn structures comprises: a first plurality of shape memory yarn structures oriented in a first direction; and a second plurality of shape memory yarn structures oriented in a second direction, different from the first direction.

Example 17: The textile of example 16, wherein the first plurality of shape memory yarn structures has a first elasticity, and wherein the second plurality of shape memory yarn structures has a second elasticity, different from the first elasticity.

Example 18: The textile of example 16, wherein the first plurality of shape memory yarn structures is configured to produce a first actuation force in response to a transformation temperature transition, and wherein the second plurality of shape memory yarn structures is configured to produce a second actuation force in response to a transformation temperature transition, different from the first actuation force.

Example 19: The textile of any of examples 14 to 18, wherein the plurality of shape memory yarn structures defines a plurality of shape memory coils, and wherein each shape memory yarn structure has a coil direction.

Example 20: The textile of any of examples 14 to 19, wherein each shape memory yarn structure comprises a plurality of yarns plied into the respective shape memory yarn structure.

Example 21: The textile of any of examples 14 to 20, wherein the plurality of shape memory yarn structures comprises at least one of woven tows or knitted loops.

Example 22: A system includes one or more shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, wherein the plurality of microfilaments comprises a shape memory alloy, and wherein the shape memory alloy is configured to undergo a phase transformation in response to heating above a transformation temperature; and a current source coupled to the one or more shape memory yarn structures, wherein the current source is configured to send an actuation signal to the one or more shape memory yarn structures to heat the one or more shape memory yarn structures above the transformation temperature of the shape memory alloy.

Example 23: The system of example 22, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers.

Example 24: The system of example 22 or 23, wherein the one or more of shape memory yarn structures defines one or more shape memory coils, and wherein each shape memory yarn structure has a coil direction.

Various examples of the disclosure have been described. Any combination of the described systems, operations, or functions is contemplated. These and other examples are within the scope of the following claims. 

1: A shape memory coil comprising: a coiled shape memory yarn having a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy. 2: The shape memory coil of claim 1, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are different. 3: The shape memory coil of claim 1, wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same. 4: The shape memory coil of claim 1, wherein the plurality of microfilaments includes between about 10 and about 1000 microfilaments. 5: The shape memory coil of claim 1, wherein the shape memory alloy comprises at least one of a nickel-titanium alloy or a copper-zinc-aluminum alloy. 6: The shape memory coil of claim 1, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers. 7: A method for manufacturing a shape memory coil, comprising: coiling a shape memory yarn to form a coiled shape memory yarn that has a coil direction around a coil axis, wherein the coiled shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy. 8: The method of claim 7, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers. 9: The method of claim 7, further comprising twisting the plurality of microfilaments to define the twist direction. 10: The method of claim 7, wherein the shape memory yarn is coiled around a mandrel to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are opposite. 11: The method of claim 10, further comprising heat treating the yarn to torque balance the yarn. 12: The method of claim 7, wherein the yarn is coiled to create a torsional imbalance to define the coil direction, and wherein the coil direction of the yarn and the twist direction of the plurality of microfilaments are the same. 13: The method of claim 12, further comprising plying the shape memory yarn with one or more shape memory yarns to torque balance the shape memory yarn. 14: A textile, comprising: a plurality of interlocked tows, wherein at least a portion of the plurality of interlocked tows comprises a plurality of shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, and wherein the plurality of microfilaments comprises a shape memory alloy. 15: The textile of claim 14, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers. 16: The textile of claim 14, wherein the plurality of shape memory yarn structures comprises: a first plurality of shape memory yarn structures oriented in a first direction; and a second plurality of shape memory yarn structures oriented in a second direction, different from the first direction. 17: The textile of claim 16, wherein the first plurality of shape memory yarn structures has a first elasticity, and wherein the second plurality of shape memory yarn structures has a second elasticity, different from the first elasticity. 18: The textile of claim 16, wherein the first plurality of shape memory yarn structures is configured to produce a first actuation force in response to a transformation temperature transition, and wherein the second plurality of shape memory yarn structures is configured to produce a second actuation force in response to a transformation temperature transition, different from the first actuation force. 19: The textile of claim 14, wherein the plurality of shape memory yarn structures defines a plurality of shape memory coils, and wherein each shape memory yarn structure has a coil direction. 20: The textile of claim 14, wherein each shape memory yarn structure comprises a plurality of yarns plied into the respective shape memory yarn structure. 21: The textile of claim 14, wherein the plurality of shape memory yarn structures comprises at least one of woven tows or knitted loops. 22: A system, comprising: one or more shape memory yarn structures, wherein each shape memory yarn structure comprises one or more shape memory yarns, wherein each shape memory yarn comprises a plurality of microfilaments having a twist direction around a yarn axis, wherein the plurality of microfilaments comprises a shape memory alloy, and wherein the shape memory alloy is configured to undergo a phase transformation in response to heating above a transformation temperature; and a current source coupled to the one or more shape memory yarn structures, wherein the current source is configured to send an actuation signal to the one or more shape memory yarn structures to heat the one or more shape memory yarn structures above the transformation temperature of the shape memory alloy. 23: The system of claim 22, wherein the plurality of microfilaments has an average diameter less than about 10 micrometers. 24: The system of claim 22, wherein the one or more of shape memory yarn structures defines one or more shape memory coils, and wherein each shape memory yarn structure has a coil direction. 